In 1999, Kasai won the ski jumping competition at the Holmenkollen Ski Festival and Nordic Tournament overall title. He collected a total of seven medals at the Nordic World Championships, including two silver (team large hill in 1999 and 2003) and five bronze medals (individual normal hill and individual large hill in 2003, team large hill in 2007 and 2009, and mixed team normal hill in 2015). At the 2010 Winter Olympics in Vancouver, he finished eighth on the large hill and 17th on the normal hill.