He joined Nippon Steel Corporation Kimitsu Works, a team in the Japanese industrial leagues, upon graduating high school in 1991.

Matsunaka made his debut at the ichigun (Japanese equivalent of "major league") level in 1997, his rookie season, as the starting first baseman and No.

Matsunaka had a breakout year in 2000, hitting .312 with 33 homers and 106 RBI and winning the Pacific League Most Valuable Player award for the first time in his career.

[2] However, while the Hawks won the league title and reached the Japan Series for the second straight year, they blew a commanding 2-0 lead to the Yomiuri Giants and lost in six games.

The Hawks had a particularly potent lineup that year, with Matsunaka, Kokubo (44), catcher Kenji Johjima (31), and second baseman Tadahito Iguchi (30) all hitting 30 or more home runs.

However, while the Hawks broke a franchise record by hitting 203 home runs as a team, they finished second to the Osaka Kintetsu Buffaloes in the pennant race, missing their third straight league title.

Matsunaka struggled to adjust to the new strike zone that the NPB had decided to implement in the 2002 season, hitting a career-low .260 with 28 home runs.

Despite the high expectations placed upon the new trio of Matsunaka, Kokubo (who had returned to the Hawks after a stint with the Giants via free agency) and newly acquired Hitoshi Tamura that would comprise the middle of the order, all three missed significant playing time due to injuries.

In 2006, Matsunaka was chosen to the national team for the third time, playing in the inaugural World Baseball Classic as Japan's starting left fielder and cleanup hitter.

Matsunaka is a burly 183 cm (6 ft), 97 kg (214 lb) power hitter[19][20] who currently hits in either the 3-hole or the cleanup spot in the Hawks' lineup.

Though he won the Pacific League Golden Glove award at first base in 2004, many thought that he was chosen over other candidates with better defensive reputations, such as Marines first baseman and three-time Golden Glove winner Kazuya Fukuura, largely on merit of his offensive production (Matsunaka led the league in all three Triple Crown categories that year).

He has since seen an increasing number of starts as the team's designated hitter due to a history of shoulder- and elbow-related injuries.