Eleven samples from six dike swarms in Southwest Japan were dated by the K-Ar method. Most K-Ar ages obtained for these samples indicate that a volcanism as dike intrusions occurred about 15 Ma ago at some areas in the inner zone of Southwest Japan. The previous geochronological studies by other workers have already revealed the occurrence of granitic intrusions and their related effusive rocks at about 14 Ma ago in the outer zone of Southwest Japan. Hence, it is inferred that the igneous activity was intensive in the relatively wide regions of Southwest Japan about 15 Ma ago.