Health, Labor and Welfare Minister Chikara Sakaguchi told a Diet committee Thursday that he did not participate in the national pension scheme in 1984 and 1985.
Lawmakers were not required to join the scheme until April 1986.
Sakaguchi, who oversees the national pension system, had earlier only stated that he was in the scheme and had paid premiums from April 1986 to April 1994, when he turned 60 and no longer had to participate.
Japanese aged from 20 to 59 are generally required to participate in the national pension plan and to pay premiums.
Prime Minister Junichiro Koizumi has also admitted that he did not participate in the scheme for a time when it was not mandatory.
As a result of the ongoing pension scandal, Yasuo Fukuda has been forced to step down as chief Cabinet secretary and Naoto Kan forced to quit as president of the Democratic Party of Japan.
Koizumi has stated that no one has to take responsibility for failing to join the scheme before it became mandatory in 1986.
Yet DPJ Acting President Ichiro Ozawa, who also failed to join the national pension plan before 1986, withdrew his bid for the presidency of the largest opposition party as a result of this failure.