Former Defense Minister Nobuo Kishi plans to resign as a lawmaker, sources close to the matter said Saturday, likely setting up a by-election in April at the same time one is held for a seat previously held by his slain brother, former Prime Minister Shinzo Abe.

Kishi, currently an adviser to Prime Minister Fumio Kishida, has reportedly dealt with health problems in recent years and his decision not to seek re-election was disclosed at a meeting of his supporters in Iwakuni, Yamaguchi Prefecture, last month.

The ruling Liberal Democratic Party is expected to field the 63-year-old's eldest son Nobuchiyo in the Yamaguchi No. 2 constituency of the Lower House of parliament, while soliciting candidates for the No. 4 district that has been vacant since Abe was assassinated last July.

Shinji Yoshida, a city assemblyman in Shimonoseki, has already announced his bid for Abe's former seat. The 38-year-old has secured backing from Abe's widow, Akie, and local supporters of the former leader.

A source within the LDP's Yamaguchi chapter said Kishi, a grandson of former Prime Minister Nobusuke Kishi, is likely to resign as a Lower House member in February.

Abe's younger brother was first elected to parliament in 2004 as an Upper House member. He was then elected to the Lower House in 2012 and is currently in his fourth term. He was defense minister until Kishida reshuffled his Cabinet last August.