Next, as the epoxy curing agent (B) used in the present invention, those represented by the following formula (I) can be referred to: wherein R1, R2, R3 and R4 are independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or an aryl group having 6 to 10 carbon atoms, provided that at least one of R1, R2, R3 and R4 is an alkyl group having 1 to 4 carbon atoms or an aryl group having