TOKYO (REUTERS) - Japan's largest opposition Democratic Party has demanded that the head of a school operator with ties to Prime Minister Shinzo Abe's wife appear before parliament to explain its purchase of government-owned land.

The operator, Moritomo Gakuen, last year bought an 8,770-sq-m plot of government-owned land for 134 million yen (S$1.7 million), or 14 per cent of its appraisal price, to build a new elementary school, Kyodo news agency reported.

Akie Abe is set to be honorary principal of the new school in Japan's western region of Toyonaka, Moritomo Gakuen says on its website.

Prime Minister Abe told parliament on Friday that neither he or his wife had been involved in the transaction.

Although the appraisal price of the land was 956 million yen, an estimated cost of 822 million yen for land cleanup activities was deducted from the sale price, Kyodo said.

"The focus is whether the cost estimate of 800 million was truly right," Democratic Party lawmaker Yuichiro Tamaki said, following a visit to the site and meetings with finance ministry officials the previous day.

"During the field trip, no evidence has emerged to support construction work worth 800 million yen was actually carried out or such work was necessary in the first place," he told reporters.

No officials of the finance ministry or Moritomo Gakuen were immediately available for comment on the land transaction.

Fellow Democratic Party lawmaker Kiyomi Tsujimoto told the same news conference that the party was demanding that Yasunori Kagoike, the president of Moritomo Gakuen, appear before a parliamentary committee to give an explanation of the purchase.

Moritomo Gakuen has been in the media spotlight in recent weeks following online comments by its kindergarten that domestic media described as possible hate speech against Koreans and Chinese people, for which the school then apologised.

Kagoike heads the Osaka branch of Nippon Kaigi, or Japan Conference, a nationalist lobby group with close ties to Abe and his cabinet.