Among the above-mentioned diamine compounds of the present invention, the diamine compounds represented by the following formula (II) are preferable: ##STR6## wherein R3 and R4 each is an alkyl group having 1 to 6 carbon atoms; R1 and R5 each is hydrogen or an alkyl group having 1 to 6 carbon atoms; Ar2 is a bivalent phenyl group or a bivalent group selected from the group consisting of a biphenyl