An optical device includes a small and low-cost variable polarization plane rotator that can control a rotation angle of the polarization plane easily. A variable polarization plane rotator is provided with a λ/4 phase plate having an optical axis in the same direction as, or at a 90 degree angle relative to, a polarization direction of an input light beam. A phase difference variable element has an optical axis at a ±45 degree angle relative to the optical axis of the λ/4 phase plate, to apply a variable phase difference between the polarization components parallel to and perpendicular to the optical axis thereof. A phase difference adjustment section adjusts the variable phase difference of the phase difference variable element, wherein the input light beam after being transmitted through the phase difference variable element to form elliptically polarized light or circularly polarized light, is transmitted through the λ/4 phase plate to form linearly polarized light. The polarization plane of the input light beam is rotated by an angle corresponding to the phase difference applied by the phase difference variable element.