Yukio Hatoyama, former president of the Democratic Party of Japan, and three other DPJ lawmakers have admitted they failed to pay into the national pension program during certain periods.

Hatoyama said on his Web site that he failed to pay pension premiums from 1986, when he was first elected to the House of Representatives, up until 1997 when he realized he had not paid.

He said that after the discovery, he made back payments for the 1996-1997 period.

During the period that he failed to pay, Hatoyama played a major role in politics, holding posts as deputy chief Cabinet secretary under then Prime Minister Morihiro Hosokawa and secretary general of New Party Sakigake, which has since disbanded.

"I did not know that it was necessary to pay into both the pension program for lawmakers and the national pension program. I am sorry for having been careless," Hatoyama said on his Internet site.

Tetsuro Fukuyama and Ryuji Yamane, both DPJ members of the House of Councilors, also said on their Internet sites that since becoming lawmakers, there were periods when they failed to pay premiums.

Lower House member Naoki Minezaki, head of the DPJ's tax council, said Friday he has failed to make payments. Naoto Kan, leader of the DPJ, admitted Wednesday he did not pay into the pension program for 10 months in 1996, when he was health and welfare minister.