That is, the compound is a mixture comprising at least one compound selected from the group consisting of urethane acrylate compounds represented by the following formula (I): ##STR1## wherein R1 is a hydrogen atom or a methyl group, a is 1 or 2; A1 is a divalent or trivalent unsubstituted or substituted hydrocarbon group; b is an integer of 3 to 6; c is an integer of 3 to 14; and d is 2 or 3; pro