PURPOSE: To shorten the hardening time of the resin composition and to suppress its hardening heat-generating temperature to be low by a method wherein methyltetrahydrophthalic anhydride is used as a hardening agent and diazobicyclo undecyne is used as a hardening promotor.

CONSTITUTION: A resin composition for cast molding uses an epoxy resin, methyltetrahydrophthalic anhydride and diazobicyclo undecyne as essential components. In the resin composition for cast molding, other components such as an organic filler, a coupling agent, an antifoaming agent, a flame retarder and the like can be added and mixed. Silica, alumina, aluminm hydroxide, talc, calcium carbonate or the like can be enumerated as the inroganic filler. Thereby, it is possible to obtain the resin composition, for cast molding, which maintains an excellent electric characteristic even when its hardening time is shortened and its hardening heat-generating temperature is suppressed to be low.