- Bats Right, Throws Right
- Height 5' 10", Weight 165 lb.
- High School Tanabe High School
The Seibu Lions signed Nishioka in 1979 and he came up three years later, hitting .311/.391/.557 in 72 plate appearances and 39 games in 1982. Splitting time with Takanori Okamura in the 1982 Japan Series, he went 2 for 6 with two walks and a home run as Seibu beat the Chunichi Dragons.
Nishioka played 102 games in 1983, getting 203 plate appearances and hitting only .218/.294/.371. He was 0 for 1 in the 1983 Japan Series, in which Seibu beat the Yomiuri Giants. He improved to .266/.314/.430 with 7 homers in 207 AB in 1984. He put up a .243/.359/.364 in 1985. In the 1985 Japan Series, he was 1 for 3 with a walk and a homer to help Seibu beat the Hanshin Tigers.
On August 6, 1986, Nishioka joined with Kazuhiro Kiyohara, Hiromichi Ishige, George Vukovich, Koji Akiyama and Takuji Ota to set a Nippon Pro Baseball record for homers in an inning. He had 243 plate appearances over 109 games for the year, with 10 homers in 208 AB and 7 steals in eight tries. Despite being a part-timer, he won a Gold Glove in the outfield. He was 3 for 13 in the 1986 Japan Series, as Seibu beat the Hiroshima Carp.
Tsuyoshi hit .247/.275/.390 in 93 games in 1987; he played two games in the 1987 Japan Series but did not bat as Osamu Abe, Akiyama and Vukovich got the playing time in the outfield and Seibu beat Yomiuri. In 93 plate appearances in 1988, Nishioka hit an excellent .356/.380/.540. He sat out the 1988 Japan Series, in which the Lions beat the Dragons for his 5th title.
Nishioka hit .206/.261/.265 in 62 games in 1989, his last year with Seibu. He moved to rival Yomiuri in 1990 and hit .249/.297/.391 in his familiar platoon role. Facing his old team in the 1990 Japan Series, he walked in his lone plate appearance as Yomiuri fell. Warren Cromartie, Sadaaki Yoshimura and Tatsunori Hara were the Giants' starting flyhawks.
Nishioka put up a .236/.333/.360 line in 1991, .231/.278/.374 in 1992, a meek .145/.217/.210 in 1993 and just 2 for 23 with two walks in 1994. He played two games but did not bat in the 1994 Japan Series, in which Yomiuri beat Seibu. He wrapped up in 1995 with the Chiba Lotte Marines, going 2 for 22 with a walk.
Overall, Nishioka batted .245/.280/.391 in 992 games in NPB. He later coached for the Marines and the Giants.