Ishiba Shigeru has been re-elected Prime Minister of Japan, following a runoff vote in the Diet's Lower House, the first of its kind in 30 years.

Ishiba, who leads the main ruling Liberal Democratic Party, won in the second vote, beating Constitutional Democratic Party leader Noda Yoshihiko.

The LDP and its junior coalition partner, Komeito, lost their majority in last month's Lower House election. But the LDP is still the largest party in the House, and opposition parties failed to unite behind one candidate.

Ahead of the voting, the LDP's Nukaga Fukushiro was chosen to return to his role as the chamber's speaker.

Ishiba is now expected to announce his new Cabinet. While most ministers will stay on, a few newcomers will be named to replace those who lost their seats.

The Emperor will ceremonially endorse Ishiba and his chosen Cabinet at the Imperial Palace later on Monday. After that ceremony, Ishiba is expected to hold a news conference to discuss his next moves.