An apparatus for detecting a displacement of a diffraction grating set on a measured object, comprises a light-emitting device, a separation optical element for separating a beam from the light-emitting device into a plurality of beams, a multiplexing optical element for multiplexing light beams diffracted by the diffraction grating from two beams out of the plurality of beams, a first detecting unit for detecting interference light generated when the beams of diffracted light are multiplexed by the multiplexing optical element and outputting a signal related to relative displacement information to the diffraction grating, and a second detecting unit for outputting a signal related to different information from the relative displacement information to the diffraction grating, based on a change in a diffracted light quantity of diffracted light by the diffraction grating from another beam out of the plurality of beams and/or another diffracted light by the diffraction grating from the two beams.