Ishiba, 67, won the leadership of the ruling Liberal Democratic Party (LDP) on Friday and was expected to be sworn in as prime minister on Tuesday.
Ishiba is finalizing plans to dissolve parliament on October 9 ahead of general elections on October 27, public broadcaster NHK, four main national dailies, and other media reported, without citing sources.
On Friday, Ishiba said he wanted to call an election to shore up his mandate “as soon as possible” but declined to say when.
Former defense minister Ishiba replaced Fumio Kishida as leader of the LDP, which has governed Japan almost uninterrupted for decades.
The unpopular Kishida resigned in the wake of a damaging party slush fund scandal which added to his unpopularity among voters angry at rising prices.