A thermosetting resin represented by the formula (17), wherein ???X??? is represented by the formula (18) in which R1, R2, R7 and R8 may be the same or different and are a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group, R3, R4, R5 and R6 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group