. . . . "A method of preparing the porphyrin represented by the general formula (4) according to claim 10 , comprising; mixing a poly(imidazolylporphyrin) having a mercapto group at one end thereof, which is represented by a formula (f): wherein each of R1, R2, Im including R3 and M has the same meaning as those defined in claim 1 , respectively; and n is an integer of 0 or more, and a imidazolylporphyrin-" . .