The effect of dissociation of an ideal dissociating gas on blast waves is investigated. Quasisimilarity assumption which has been developed by OSHIMA in studying blast waves of a perfect gas without dissociation is applid to the problem of line source explosions. It is shown here that the basic equations governing the flow field behind shock waves can be computed in a similar way to the flow of a perfect gas by introducting an isentropic index in the present analysis. The results show that for dissociating gases the profiles of pressure, density and particle velocity decrease rapidly near the shock front in comparison with those for a perfect gas. For comparatively strong shock waves the isentropic index varies significantly through the analysis.