PURPOSE: To provide a plastic optical element which obviates the deformation of an optical surface at the time of mounting and in a cooling process and has high accuracy.

CONSTITUTION: This plastic optical element has an optical surface forming part 1 which has the optical surface 1a to reflect light or to allow the transmission thereof and a reinforcing part 2 which is formed around this optical surface forming part 1 and has a rib part 2a projecting to at least one surface side of the optical surface forming part 1. The plastic optical element described above is provided with a strain absorbing part 3 formed with a recessed groove 3a on the surface formed with the optical surface 1a between the optical surface forming part 1 and the reinforcing part 2.