To provide a melt tension improver for a polyolefin which can improve the processibility of a polyolefin resin and a polyolefin resin composition containing the same.

This melt tension improver for a polyolefin resin comprises a multiphase-structure-type graft copolymer which consists of polyolefin resin segments and vinyl polymer segments formed from a reaction precursor having a wt. average mol.wt. of 500,000-10,000,000. One kind of the above two kinds of segments forms a disperse phase as fine particles having particle sizes of 0.001-10 μm in the other kind of segments.