Japan's Finance Ministry announced on Monday that it will punish 20 officials including its head Taro Aso after finding that some of its documents related to the sale of heavily discounted state-owned land for a school building project were altered and discarded.

Aso will "voluntarily" return one-year's income as finance minister. At a news conference on Monday, he apologized for the controversy over the documents but claimed he had not behaved improperly.

Despite opposition parties' calls for him to resign, Aso wants to stay in office.

The Osaka-based Moritomo Gakuen, which bought the land at an 86 percent discount, is at the center of a favoritism scandal that has rocked Prime Minister Shinzo Abe's administration. His wife, Akie, had at one point been named honorary principal of the now-scrapped primary school and was said to have praised the project.

The ministry's Kinki local finance bureau drew up the roughly 4,000-page documents in 2015 and 2016 on the approval of the Moritomo Gakuen deal as well as attached papers. The originals included passages pointing out the "exceptional" nature of the deal, among other unusual references. However, these phrases had been erased or changed in documents submitted to parliament members after the land sale came under scrutiny starting in February 2017. References to Akie Abe and some influential politicians from the ruling Liberal Democratic Party were also erased.

Opposition parties grilled the government over whether Abe and his wife were involved in the land deal.

Abe had said that if he or his wife were found to have been involved in the transaction, he would resign as prime minister and parliament member.

Still, Abe has been dogged by allegations he used his influence in the approval process to open Japan's first veterinary department in half a century in a national strategic special economic zone in Imabari city, Ehime prefecture, under a university run by Abe's longtime friend Kotaro Kake.

An opinion poll conducted by the Mainichi Shimbun on May 26-27 showed that the approval rate for the Abe Cabinet was 31 percent, with disapproval standing at 48 percent.