From 1997 to 2007, Kobayashi played in the Nippon Professional Baseball league for the Chiba Lotte Marines.

[1] He earned his 200th career save in 2006, becoming the third player in Japanese baseball to reach the mark, after Kazuhiro Sasaki and Shingo Takatsu.

[2] In 2007, his final season with the Marines, Kobayashi finished 2–7 with 27 saves and a 3.61 ERA in 49 relief appearances.

[4] On November 2, 2007, Kobayashi declared free agency and expressed his interest in playing for a major league club for the 2008 season.

[4] On November 20, Kobayashi signed a two-year, $6.25 million deal with the Cleveland Indians.