The present invention provides a modifier for resins which comprises a dispersant (I) or a coatability improver (II) comprising a modified polyolefin (A) and a filler in combination, the modified polyolefin (A) containing a hydrophobic group comprising a polyolefin (a) having a number average molecular weight of 1,500 to 100,000 and 0.2 to 10 double bonds per 1,000 carbons at the molecular terminus and/or within the molecule, and a hydrophilic group containing at least one onium base (b), and a number average molecular weight per (b) being 750 to 50,000.