A polymer scale preventive agent for use in polymerization of a monomer having an ethylenically unsaturated double bond, comprising (A) a N-substituted leucophenothiazine having the general formula (I): ##STR1## wherein Z is a carbonyl group or a sulfonyl group, R1, R2 and R3 are a hydrogen atom, alkyl group, an amino group, etc., and (B) a water-soluble basic polysaccharide.