A wavelength locker includes: a birefringent plate for separating light emitted from a light source into two beams of light which are approximately in parallel to each other; a first light receiving element for directly receiving one of the two beams of light separated by the birefringent plate; a λ/4 plate and an etalon filter through which other one of the two beams of light separated by the birefringent plate passes; and a second light receiving element for receiving the other one of the two beams of light which has passed through the etalon filter. The λ/4 plate has an optic axis tilted at 45 degrees with respect to a polarization direction of light incident on the λ/4 plate, changes a polarization direction of light, which has been reflected by the etalon filter and incident on the λ/4 plate, such that the reflected light does not return to the light source after passing through the birefringent plate, and emits the resultant light.