Conventionally, a maleic acid-modified polypropylene oligomer has been used as a dispersant of clay in resins, and a hydroxy-modified polypropylene containing a hydroxyl group has been known as a resin modifier for improving the coatability of resins. But a dispersant or coatability improver having the clay dispersion effect and coatability improving effect for resins with as small amount of addition without decreasing the resin properties has not been known. The present invention provides a modifier for resins which comprises a dispersant (I) or a coatability improver (II) comprising a 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, and a filler in combination.