When you join the Health Insurance Society, you must begin paying insurance premiums. 
Insurance premiums are set according to the income (total remuneration including salary and bonuses, etc.) of each insured person. However, because the income of insured persons changes from month to month, it would be difficult to calculate insurance premiums on the basis of the actual amount received. For this reason, provisional remuneration scales divided into easily calculable units (called "standard monthly remuneration") have been established. Each insured person's salary, etc. is applied to this scale, and insurance premiums are then calculated accordingly.
Bonuses are rounded down to the nearest 1,000 yen (the result is referred to as a "standard bonus amount") for the purposes of calculating insurance premiums.

How insurance premiums are calculated

Insurance premiums are calculated by applying our Health Insurance Society's insurance premium rates to standard monthly remuneration and standard bonus amounts.
See here for our Health Insurance Society premium rates and table of monthly insurance premiums. >> "Table of monthly insurance premiums"

Points at which standard remuneration is determined

Point of employment (point of acquisition of insurance status)

Workers join Health Insurance Societies upon employment. Health Insurance Societies use starting salaries, etc. as the basis for calculation of standard monthly remuneration amount.

July 1 of each year (fixed point)

Health Insurance Societies revise standard remuneration for all insured persons once a year. Remuneration calculations are revised every year on July 1, based on the salaries, etc. earned by workers in April, May, and June. The revised figures are applied for one year, from September 1 to August 31 of the following year.

Point of substantial salary change including pay raises (whenever revision becomes necessary)

Health Insurance Societies will provisionally revise standard remuneration in cases where monthly salaries grow substantially (at the rate of two levels or more) as a result of increases in basic monthly pay or periodic wage increases.

Upon the end of childcare or other leave (revision on end of childcare or other leave)

When an insured person caring for a child less than three years of age on the date childcare or other leave ends has experienced a change of one or more grades in average monthly remuneration over the three-month period starting at the end of the leave, due to use of reduced working hours or a similar program, he or she may apply to have his or her standard monthly remuneration revised.

End of maternity leave (revised upon end of leave)

When an insured person caring for a child covered by maternity leave on the date maternity leave ends has experienced a change of one or more grades in average monthly remuneration over the three-month period starting at the end of the leave, due to reduced working hours or other reasons, he or she may apply to have his or her standard monthly remuneration revised.

Types of insurance premiums

There are three types of health insurance premiums: general insurance premiums, regulation insurance premiums, and long-term care insurance premiums. Premiums are determined by multiplying standard monthly remuneration amount and standard bonus amount by premium rates.

General insurance premiums (base premiums + specific premiums)

General insurance premiums are collected primarily to defray insurance benefits. They also serve as a revenue source to cover the costs of providing health care to elderly persons.
To clarify the degree of assistance provided to elderly persons, general insurance premiums are classified as base premiums or specific premiums.

Base premiums: premiums applied to payment for medical care, insurance service, etc.

Specific premiums: premiums applied to costs associated with supporting medical care for persons in the latter stage of old age and benefits for persons in the earlier stage of old age

Health Insurance Societies are permitted to set the rate of general insurance premiums, they deem appropriate within the range of 30/1000 to 130/1000. Burden ratios paid by insured persons and their employers may also be determined independently, based on conditions at the respective Health Insurance Societies.

Regulation insurance premiums

Health Insurance Societies across the country are pursuing joint efforts to cover high-cost medical expenses and to support Health Insurance Societies in financial difficulty (by providing subsidies for financial adjustment). Regulation insurance premiums serve as a revenue source for these activities.
The rate of regulation insurance premiums is determined by multiplying 0.0013 of the basic premium rate by the rate of slight variation (i.e., the rate of adjustment) based on the relevant Health Insurance Society's financial state.

Long-term care insurance premiums

Long-term care insurance premiums are charged for long-term care insurance. Although the long-term care insurance system is operated by municipal governments throughout Japan, each medical insurer is required to collect premiums for insured persons aged 40 - 64 and their dependents (both of these groups are referred to as category 2 insured persons under long-term care insurance). The Health Insurance Society collects long-term care insurance premiums from insured persons and their dependents aged between 40 and 64 years.
See here for more details. >> "Long-term care insurance premiums #Category. 2 insured persons (persons aged between 40 and 64 years)"

You are exempt from paying insurance premiums during child-care leave

To reduce  financial burdens while taking childcare or similar leave, Health Insurance Societies exempt insured persons and their employers from the payment of insurance premiums during the period of such leave in cases where employers have applied for such exemption. This exemption applies from the month in which the leave starts through the month immediately preceding the month that includes the day following the end of the leave.

For childcare or similar leave beginning on or after October 1, 2022, even if the days on which leave starts and ends fall in the same month, payment of insurance premiums will be waived if the leave was taken for 14 or more days during the month that includes the leave start date.
Insurance premiums on bonuses (i.e., insurance premiums on bonuses paid in the month the last day of which is included in the period of childcare or similar leave) are also waived. However, for childcare or similar leave beginning on or after October 1, 2022, this waiver is available only if the childcare or similar leave is taken for a continuous period of longer than one month that includes the last day of the month of the bonus.

Insured persons can also apply for exemption from the requirement to pay insurance premiums during the period of maternity leave, from the month including the starting date of maternity leave through the month before the month including the day after the ending date of the leave.