PURPOSE: To provide the plastic lens for an optical instrument which is high in dimensional accuracy and free from any deformation of curved surfaces due to heat, by a method wherein the outside periphery of an annular edge part provided on the outside of an effective diameter of a lens and extending on both sides or one side of the lens in the direction of the optical axis of the lens is machined for core making work.

CONSTITUTION: The annular edge part 13 extending on both sides or one side of a lens in the direction of the optical axis of the lens is provided while leaving a sufficient allowance 7 on the outside of the effective diameter 6 of the lens, and the outside periphery 8 of the edge part 13 is machined for core making to obtain the plastic lens 10.

EFFECT: Since cutting heat at the time of machining for core making is generated at a part on the outside of the effective diameter 6 of the lens, thermal deformation of the curved surface within the range of the effective diameter 6 can be perfectly prevented. When combining a plurality of lenses, the spacing between the lenses can be maintained with high accuracy without using any other component part. Further, inclination of the lens can be reduced.