Born in Kurashiki in the Okayama Prefecture, Ishii began his career with Fukuyama Rosas Seleson, where he played until he was twelve, before joining the youth team of professional side Shonan Bellmare in 2018.
[1] In August 2022, he was registered as a Type 2 player, before going on to make his debut the following season, in a 3–0 J.League Cup win against Shimizu S-Pulse.
[2][3] On 17 February 2023, Shonan Bellmare announced that Ishii would be promoted to the first team ahead of the 2024 season.
[5] Ishii was called up to the Japanese under-17 team multiple times in 2022, including scoring twice and getting an assist in a 6–1 win over Comoros.
[6] He was called up to the under-19 team for the 2023 Maurice Revello Tournament in France, and scored in Japan's opening game against Morocco, as they won 2–1.