The plastic optical element has such a characteristic that it will gain a mass increase of no more than 0.1 mass % upon moisture absorption when it is left to stand for 30 days in an atmosphere with a temperature of 40.degree. C. and a relative humidity of 20% and then left to stand for an additional 30 days in an atmosphere with a temperature of 40.degree. C. and a relative humidity of 90%. The plastic optical element includes a plastic substrate and a multi-layered film covering the plastic substrate. The multi-layered film includes a lower inorganic layer formed over the plastic substrate and an upper organic layer formed over the inorganic layer.