To provide a producing method of a plastic lens made of a polyolefin resin having small double refractivity.

In the producing method of the plastic lens, first, a polyolefin resin 1 is put in a die. Then the polyolefin resin housed in the die is heated to a temp. higher than the glass transition temp. of the resin and then held at that temp. for a specified time to be molded. Then the molded polyolefin resin is slowly cooled to about its glass transition temp. and then rapidly cooled. Thus, the orientation of molecules in the lens can be avoided and the obtd. plastic lens has small double refractivity.