An aspect of the present invention relates to a method of manufacturing a polarizing plastic lens, which comprises a reversing step of reversing a convex surface shape into a concave surface shape by pressing a polarizing film that has been curved surface processed, a casting mold assembling step of assembling a casting mold having a cavity within which the reversed polarizing film is positioned with the reversed polarizing film, an upper mold, a lower mold, and a sealing member that maintains a spacing between the upper mold and the lower mold, and a polymerizing and curing step of casting a curable composition into the cavity, and polymerizing and curing the curable composition that has been cast to provide a polarizing plastic lens within which a polarizing film is positioned.