This invention provides a displacement measurement device, a displacement measurement method, and a thickness measurement device capable of easily ensuring a conjugate relationship between the light source and the diaphragm and capable of accurately measuring the change in distance with the testing target. In the displacement measurement device, the light from the laser diode is collected towards the pin hole of the diaphragm plate at the collective lens, and then sent to the objective lens through the pin hole. The light is reflected at a surface of work, and detected by a photodiode through the objective lens, the pin hole, the collective lens, and the half mirror. That is, the pin hole becomes a substantial light source, and becomes a diaphragm with respect to the incident light on the work. The spot diameter collected on the pin hole by the collective lens is greater than the diameter of the pin hole. The light receiving quantity signal of the return light component on a diaphragm plate, shown with an outlined arrow, is removed by a high-pass filter.