A mercapto-substituted imidazolylporphyrin metal complex monomer represented by a general formula (1): ##STR1## wherein R.sub.1 represents a group selected from the group consisting of an alkyl group, unsubstituted aryl group, alkyl-substituted aryl group and alkyloxy-substituted aryl group, M represents a metal ion selected from the group consisting of Zn(II), Ga(III), Fe(II), Co(II), and Ru(II), X represents a divalent linking group containing at least one group selected from the group consisting of an arylene group and an alkylene group, R.sub.2 represents a hydrogen atom or an acetyl group, and Im represents Im.sub.1 or Im.sub.2 set forth below: ##STR2## wherein R.sub.3 represents a hydrogen atom or an alkyl group.