a plastic optical element with good dimensional accuracy and no strain is produced by providing a mold having a cavity corresponding to a shape of the optical element and a feed passage which communicates with said cavity , at least one side of which mold for forming one surface of the optical element is made of a light transmitting material so that light can pass through said material into the mold cavity , charging a photocurable plastic material through said feed passage into said cavity , irradiating a central part of said cavity with light which initiates a photocuring reaction , and enlarging the light in the form of a ring until the lighted area reaches the periphery of the cavity so as to cure the entire photocurable plastic material . the light irradiation is carried out while a supplementary portion of the photocurable plastic material is maintained in said feed passage .