The Ministry of Health, Labor and Welfare has announced that starting in April it will "take action" to increase the "collection rate" of national pension premiums, specifically those for kokumin nenkin, the obligatory pension plan for the self-employed and those who otherwise don't belong to the company-supported kosei nenkin pension system. According to Tokyo Shimbun the idea is to send warning letters to individuals whose incomes are more than ¥4 million and who haven't contributed for at least 13 consecutive months.
Presumably, the next step will be for the ministry to start siezing assets. The initial criteria would target approximately 140,000 pension scofflaws. Eventually, however, they will go after everyone who hasn't paid, and since it is estimated that close to 3 million people who should be paying into the system haven't been for at least 24 months, the job seems daunting if not impossible.
There are many reasons for this delinquency, but the main one has to do with the system itself. Basic pensions apply not only to the self-employed, but anyone who is employed part-time or on a contract basis, meaning their employers don't pay into the kosei nenkin system. It also includes the unemployed, because according to the law every adult who lives in Japan must belong to the system, whether they work or not. And the premiums are the same, regardless of income or lack thereof: right now ¥15,250 a month (it goes up gradually every year).