thyl-s-triazine photopolymerization initiators or azinium salt photopolymerization initiators, and which has anabsorption coefficient of more than 1000 at wavelengths longer than 300 nm, L.sup.1, L.sup.2 and L.sup.3 are connecting groups connecting S and D, provided that S and D are not conjugated by L.sup.1, L.sup.2 and L.sup.3, l is an integer of 1 to 5, m isan integer of 1 to 5, n is an integer