Here is an unabridged version of what previous posters have stated.
In April 1986, participation in
kokumin nenkin was in principle made compulsory for all registered residents of Japan over the age of 20. If for one reason or other you missed making contributions for any period after that date, four basic designations apply:
Periods without insurance coverage (
hokenryō minō kikan 保険料未納期間)
The SOL for making mandatory contributions to
kokumin nenkin is two years. In other words, unless you fall into one of the three categories described below, you have two years to make up any missed payments. After that, the period is excluded from both eligibility for coverage and calculations of pension benefits.
Periods exempt from insurance coverage (
hokenryō menjo kikan 保険料免除期間)
These are periods during which an individual, either upon application or for specific legal reasons, is considered exempt from making pension payments. The exemption amount is either 100%, 75%, 50%, or 25% of the mandatory contribution in effect. This period is included in the overall coverage period, but pension benefits are reduced accordingly, except now in cases where maternity/paternity or childcare leave applies (in these cases, individuals are deemed to have made the mandatory contribution and benefits are not reduced). Up to 10 years of back payments are permitted for these individuals.
Periods during which pension payments have been postponed (
yūyo kikan 猶予期間)
Periods during which Category 1 insured persons (i.e., self-employed workers) under the age of 50 have, upon application due to financial hardship, been temporarily excused from making
kokumin nenkin contributions. This period is included in the overall coverage period, but no pension benefits accrue. Up to 10 years of back payments are permitted for these individuals.
Cumulative periods of deemed coverage (
gassan taishō kikan 合算対象期間; also called
kara kikan カラ期間)
This refers primarily to periods between April 1961 and March 1986 when
kokumin nenkin coverage was optional for dependent spouses and certain other individuals. The category includes those between the ages of 20 and 59 who were resident overseas or were students for any period between April 1961 and March 1991 and voluntarily suspended their contributions to the scheme (up to 1991, student participation was optional). These periods are included when considering eligibility for pension benefits, but the benefits are reduced accordingly.
N.B. Unlike
menjo kikan or
yūyo kikan, makeup contributions are NOT possible for periods of deemed coverage, although voluntary coverage is conditionally available between the ages of 60 and 64 to top off your
kokumin nenkin benefits (this voluntary period can be extended to the age of 70 if it would enable you to reach the 10-year minimum).
The National Pension Service lists five categories of deemed periods of coverage in effect since April 1986:
[See
https://www.nenkin.go.jp/service/jukyu/ ... 21-05.html]
1. Periods during which Japanese nationals resident overseas have voluntarily suspended their
kokumin nenkin contributions (only applicable between the ages of 20 and 59).
2. Periods through March 1991 during which the individual was a student and voluntarily suspended their
kokumin nenkin contributions, excluding those taking night-school courses or correspondence courses (only applicable between the ages of 20 and 59).
3. Periods for Category 2 insured persons (i.e., those covered by
kōsei nenkin) preceding the age of 20 or following the age of 60.
4. Periods during which the individual was eligible to contribute voluntarily to
kokumin nenkin but did not do so (only applicable between the ages of 20 and 59).
5. Periods during which individuals who obtained Japanese citizenship or who became permanent residents on or after May 1, 1961, resided overseas before receiving Japanese citizenship or permanent residence (only applicable between the ages of 20 and 59).
More complicated conditions apply if the period goes back to before April 1986, but this has been the situation for the past 35 years (possible corrections welcome). Individual circumstances can always complicate matters, however, so -- as others have pointed out in various threads -- it always pays to go to the local pension office yourself to get a definitive answer, even if Kiro's experience is probably representative.
