In a displacement sensor, an illumination optical system, which converts laser light emitted from an emission unit into first converging light, and irradiates with the first converging light a detection space through which a target moves. An imaging optical system converts laser light reflected from the target into second converging light, and irradiates with the second converging light a light-receiving surface of a detection unit. The image of the emission unit into which the illumination optical system focuses the first converging light is displaced along the optical axis of the imaging optical system by a temperature change of the illumination optical system. The range in which the image is displaced is limited to be located either anterior to the front principal point of the imaging optical system, or posterior to the rear principal point of the imaging optical system.