"I want the entire ministry to make efforts, including Finance Minister Taro Aso", Abe told reporters during a trip to Fukushima Prefecture.
Japan's Finance Ministry will admit altering documents pertaining to a state-owned land sale at the centre of favouritism allegations against Prime Minister Shinzo Abe, government sources said on Saturday.
Aso, who doubles as deputy prime minister, said the ministry is expected to present early next week an interim report on its internal probe into whether the alleged documents exist. Sagawa was frequently summoned to parliament previous year to explain the government's handling of the scandal, and his blunt and impatient responses sparked public criticism.
The original document described the land transaction between the ministry and the school operator as "exceptional", but this word was missing in the version made available to Diet members during debate about the scandal previous year.
The opposition has accused ministry officials of hiding some key documents and colluding to cover up the scandal.
On the day of Mr Sagawa's resignation, press reports said the police were investigating as a possible suicide the death of an official at a Finance Ministry bureau that handled the land deal. Opposition lawmakers said they would further scrutinize the scandal.
A finance ministry employee in the western region where the school is located was found dead at his home on March 7 and police are investigating it as a suicide, the Kyodo and Jiji news agencies reported.
The transaction involved a plot of land in Osaka Prefecture that was sold to Moritomo Gakuen in 2016 for around 134 million yen ($1.3 million), much less than its appraised value of 956 million yen.
The ministry has said the heavily discounted price of the land was due to the cost of removing buried waste at the site.
Moritomo Gakuen, headed by Yasunori Kagoike, had also sought donations while referring to the school as the "elementary school in honor of Shinzo Abe", prompting the premier to lodge a protest for using his name without permission.
Abe has denied that he or his wife did favors for the operator of the school, Moritomo Gakuen. Kagoike and his wife were arrested in July on suspicion of illegally receiving subsidies.