An annual cherry blossom viewing party organized by the Japanese prime minister has come under scrutiny.
The opposition Japanese Communist Party says the number of guests and cost of holding it have been increasing year after year.
It also criticized Prime Minister Shinzo Abe for inviting many of his supporters from his local constituency.
Officials from four opposition parties, including the Japanese Communist Party, agreed in a meeting on Monday that if it's true, Abe has been mixing public and private interests.
The four parties said they will set up a panel to study what's behind the increase in the number of participants.
They also agreed to demand that the government disclose its criteria for inviting guests.
The Diet affairs chief of the Constitutional Democratic Party, Jun Azumi, described the matter as "extremely grave," as public money may have been used for private purposes.
Azumi said Abe needs to explain it himself.