A thermosetting resin composition is produced by heat treating a mixture of a polyaddition thermosetting resin, a silicic compound of the formula ##STR00001## wherein R is an organic group containing a functional group that causes an addition reaction with the curing agent and R.sup.5 and R.sup.6 are independently a methyl group or an ethyl group, and water and adding a curing agent of the polyaddition thermosetting resin to the mixture that has undergone heat treatment.