To lighten a polyolefin molding without lowering mechanical properties such as strength and elastic modulus even in the case of conducting high magnification orientation.

A film containing high-density polyolefin as a main component or a sheet-like material is rolled, then oriented several times in a range of 75°C to a melting point of the polyolefin, and molecular chains are extended, thereby improving strength and elastic modulus. Further, the film is oriented in the same direction at a temperature lower by 35°C than the lowest orienting temperature in a multi-stage orienting step to a temperature lower by 5°C than the lowest orienting temperature to lower a density of the molding and to lighten it.