Opposition lawmakers say officials at the Cabinet Office discarded the list of guests invited to the prime minister's event just when they began an inquiry into the event.
Opposition parties criticize Prime Minister Shinzo Abe for inviting too many of his local supporters to the annual cherry blossom viewing party hosted by him. They say he is using the publicly-funded event to promote his own interests.
An opposition team met officials from the Cabinet Office and other government agencies on Thursday for the third consecutive day.
Opposition lawmakers asked the officials when they discarded the guest list and other related documents.
Cabinet Office officials said that the papers related to the cherry blossom viewing party held on April 13 were disposed of on May 9.
A Communist Party lawmaker said that the opposition began questioning Cabinet Office officials on May 9 to prepare questions they planned to raise at the Diet.
Other opposition lawmakers asked why they abruptly threw away the papers on that day after they had kept them for nearly a month.
The Cabinet Office officials answered that they usually discard them at around that time of the year.
Abe earlier told the Diet that the guests were chosen for their contributions to their respective fields, based on the recommendations of various ministries and agencies.
He also said that he was not involved in the selection process.
NHK has obtained letters, sent by Abe's personal office, inviting members of his constituency to take part in the cherry blossom viewing event.