The 67-year-old former defence minister, who won a close-fought contest to lead the ruling Liberal Democratic Party last week, is virtually assured of being formerly designated premier due to his party's majority in parliament.
The result of the vote is expected around 1:40 p.m. (0440 GMT) which will then be followed by the Emperor formally appointing Ishiba, and his cabinet, in a ceremony at Tokyo's Imperial Palace. Ishiba is also expected to hold a late press conference on Tuesday.
The veteran lawmaker, seen as somewhat of an outsider in his party who failed at four previous leadership bids, has already begun picking government and party officials who will contest the upcoming general election with him.
So far those include two rival candidates in the leadership race, Katsunobu Kato as finance minister and Yoshimasa Hayashi to stay on as chief cabinet secretary, a pivotal post that includes the role of top government spokesman, two sources familiar with the appointments told Reuters earlier.
A close Ishiba ally, Takeshi Iwaya, a former defence chief, will take over as foreign minister, while Gen Nakatani will return to the defence ministry, a position he held in 2016, said the sources, who asked not to be identified because they are not authorized to speak to the media, confirming earlier media reports.
Yoji Muto, a former junior minister, will take charge at the economy, trade and industry ministry, a separate source said.
Not included in his picks, however, is Sanae Takaichi, the hardline conservative he beat by 215 votes to 194 on Friday in the closest leadership election in almost seven decades.
Takaichi's absence could make it difficult for Ishiba to manage a fractious ruling group roiled by scandals that have sapped its public support.
