PURPOSE: To prepare an epoxy resin composition rapidly curable without foaming, by compounding the resin with a tetra-substituted ammonium-tetra-substituted borate compound prepared by using a specific tertiary amine, etc., as a curing accelerator.

CONSTITUTION: A tetra-substituted ammonium-tetra-substituted borate compound of the formula [R1WR4 are H, alkyl, hydroxyl or (substituted) phenyl; R5 is (substituted) phenyl] is prepared by reacting a tetraphenyl boron compound with a tertiary amine or a quaternary ammonium compound having a higher boiling point than the molding temperature (e.g. 150°C), e.g. triphenylamine, dimethylaminoethanol, etc.. The compound is added to an epoxy resin as a curing accelerator together with a curing agent