A photopolymerizable composition comprising an addition polymerizable ethylenically unsaturated compound and a photopolymerization initiator system, wherein the photopolymerization initiator system comprises:(a) a squarylium dye containing a following formula (I), ##STR7## wherein each of R1 and R2 is a substituted or unsubstituted alkyl group or aryl group, and each of A and B is a substituted or