A displacement detecting device includes a light source for projecting light to an object to be examined, the light source being movable relative to the object to be examined, a first reflecting element being fixed to the object to be examined and including a reflection portion for reflecting light from the light source and a non-reflection portion, a first light receiving element for receiving light reflected by the first reflecting element, a second reflecting element fixed to the object to be examined and including a reflection portion for reflecting light from the light source and a non-reflection portion, and a second light receiving element for receiving light reflected by the second reflecting element, wherein the second reflecting element is disposed at a light path through which at least a portion of light from the light source and advancing via the first reflecting element is directed to the second light receiving element.