Nagashima made his professional debut in April 1958, and struck out in all four of his at-bats against Masaichi Kaneda (coincidentally, Sadaharu Oh also struck out in all of his at-bats in his debut game against Kaneda). Regardless, Nagashima became the team's clean-up hitter by mid-season, and the Giants won the league championship. Nagashima led the league in HRs (29) and RBIs (92), and was awarded the rookie of the year award. He would have hit .300 with over 30 home runs and 30 steals in his rookie year, but he had one home run scratched off his record because he forgot to step on first base while rounding the bases after hitting a home run.