Lajunen, who started for the Jyväskylän Hiihtoseura club in his hometown Lieksa, began professional ski jumping in the Continental Cup (COC) in 1997 . On March 4, 1999, he was nominated for the first time in the national group for the World Cup jumping in Kuopio and was able to win his first World Cup point with 30th place. However, it should remain his only point win at a World Cup. At the Junior World Championships in 2000 in Štrbské Pleso , he won the bronze medal with the team. In the Continental Cup he was able to steadily improve his performance and was able to win the overall standings with 929 points in the 2000/01 season . Among other things, he won the jumping on the Adlerschanze in Schönwald in the Black Forest . The following season, however, was no longer successful, so that after only 51 points and 150th place in the overall standings, he ended his career as a ski jumper and switched to the Nordic Combined.
With the combined athletes, Lajunen started in January 2003 for a total of five races in the B World Cup. However, he was also unsuccessful here and therefore ended this career early after the end of the season.