This invention provides a polyolefin-based molded article containing microfibrillated plant fibers having high strength and high elastic modulus. Specifically, the present invention provides a composition comprising (1) a polymeric compound having a primary amino group, (2) a polymeric compound modified with maleic anhydride, (3) microfibrillated plant fibers, and (4) a polyolefin; and a molded article obtained by subjecting the composition to heat treatment.