The present invention is low cost, does not exert a negative influence on the surroundings, has a small group-velocity delay dispersion, efficiently blocks reflected return light such that the laser light does not return to the laser light source, and allows properly polarized laser light to enter a polarization-dependent element in a subsequent stage. An optical device is provided which includes a laser light source, an optical system that transmits laser light emitted from the laser light source, and a polarization-dependent element into which the laser light transmitted by the optical system enters. The characteristics of outgoing light are changed according to a polarization state of the incident light. The optical system includes a reflected-light generator that reflects a part of the transmitted laser light, and λ/4 wave plates are disposed such that the reflected-light generator is disposed therebetween.