The additive for a polymerizable composition according to the present invention contains a compound represented by the general formula (a). In the general formula (a), R represents a saturated hydrocarbon group having 1 to 3 carbon atoms. M represents Sn, Sb, Bi, or Ge. m represents 0 or 1. R and M are not directly bonded when m is 0. n represents an integer of 1 to 3. X represents a monovalent linking group, and a plurality of X may be the same as or different from each other. When two or more linking groups X are bonded with the metal atom M, the linking groups X may combine together to form a ring.