He debuted in the Pacific League in his rookie season of 1994, played in 78 games.
Kokubo was one of Japan's leading power hitters during the 1990s and early 2000s. He hit over 40 home runs in 2001 and 2004, but only led the league in the category once (1995), with only 28 home runs. He also led the league in RBIs in 1997.
He was suddenly given away to the Yomiuri Giants in 2003, despite being the team's cleanup hitter. Kokubo was seemingly given away for free, since the Giants did not give a player to the Hawks in exchange. The motives behind this transaction remain a mystery.[5] The Hawks replaced him during his injuries with foreigner Julio Zuleta.
He played with the Yomiuri Giants for three years before signing with his former team (now the Fukuoka SoftBank Hawks) during the 2006 off-season as a free agent.[6]
On June 24, 2012, Kokubo recorded his 2,000th career hit, becoming the 41st Japanese professional baseball player to reach the milestone.[2] On August 14 he announced his intention to retire at the end of the 2012 season.[8] His retirement ceremony was held on October 8, after the last regular-season match against the Orix Buffaloes, which SoftBank lost after being no-hit.[9]
After retirement
After his retirement, He worked for NHK as a baseball broadcast commentator.
On December 3, 2020, Kokubo became the head coach of the Fukuoka SoftBank Hawks.[10]
He had been the second squad manager until the 2023 season, when he was set to replace Hiroshi Fujimoto as the manager of the first team in 2024, after Fujimoto announced he was going to step down as manager.[1]
International career
He won a bronze medal in the 1992 Summer Olympics before entering the Japanese professional leagues.[11]