Kagayama began motorcycle racing in 1990. He competed in the All Japan Road Race Championship for many years, building his reputation in the 750cc Superbike and GP250 classes through the 1990s. He also contested four 250cc World Championship races in 1997 and 1998, finishing in the top eight in all four appearances. By 2001 he was fourth in the All Japan Superbike standings, and the following year he served as a MotoGP test rider for Suzuki.
In 2003, Kagayama joined Rizla Suzuki in the British Superbike Championship alongside double British champion John Reynolds. He had won three races when a heavy crash at Cadwell Park cut his season short. He returned in 2004 — still not fully fit early in the year after a further crash caused a broken collarbone — and nonetheless finished third in the championship.
For 2005, Kagayama stepped up to the Superbike World Championship with the Belgian-based Alstare Suzuki team, replacing the retiring Troy Corser. Racing on largely unfamiliar circuits, he won races against some of the world's best riders and finished fifth overall in his debut season — a remarkable result for a newcomer. He retained his ride for 2006. After a collision with Noriyuki Haga on the final lap of the opening race while contending for the lead, his season stalled. He recovered with a third and a fourth at Misano before delivering an outstanding double win from the second row at Brno, nearly doubling his points tally in a single weekend. He finished seventh overall that year.
Kagayama stayed with Suzuki's World Superbike programme through 2007, 2008, and 2009, though injury repeatedly interrupted his campaigns. In 2007 he missed four rounds and ended the season ranked thirteenth. He switched his racing number to 34 for 2008 — the number associated with former 500cc World Champion Kevin Schwantz — but again suffered an injury-disrupted year. He remained the lowest-placed of the three Suzuki works riders in that season's standings and finished twelfth in 2009.
Kagayama has an impressive record at the Suzuka 8 Hours endurance race. Competing consistently from 2000, his highlights include: second in 2004 with Atsushi Watanabe; outright victory in 2007 partnered with Kousuke Akiyoshi for Yoshimura Suzuki; second in 2011; and third-place finishes in 2013, 2014, and 2015, the latter two as team owner as well as rider under the Team Kagayama banner. His 2013 podium was achieved alongside former 500cc world champion Kevin Schwantz and Noriyuki Haga.
In 2010, Kagayama returned to the British Superbike Championship with Worx Suzuki, partnering Tommy Hill. He finished fifteenth overall with one podium. From 2011 onward, Kagayama shifted his focus to Japan, founding Team Kagayama to contest the JSB1000 class of the All Japan Road Race Championship. As owner and sole rider, he scored the team's first victory in 2012 at the Sugo circuit. The team continued competing into the 2010s, with Kagayama himself racing well into his forties.
Kagayama's career traces a distinctive arc: from domestic Japanese talent to a competitive World Superbike rider capable of outright wins, then to team founder sustaining his competitive involvement through a self-funded Japanese operation. His adoption of Kevin Schwantz's number 34 reflected both personal admiration and an identification with the Suzuki racing tradition. His 2007 Suzuka 8 Hours victory for Yoshimura Suzuki stands as one of the signature moments in his long career with the brand.