Optical axial displacement sensor

An optical displacement measurement device (10) for introducing light from a light source (20) to an object (2) through an optical system having an objective lens (28) and for detecting light reflected from the object by a detection section (30) to thereby measure a displacement of the object (2) relative to the objective lens. In the optical displacement measurement device, the objective lens (28) is a conical lens for advancing the light toward the object and for receiving light reflected from the object to direct to the detection section, and the displacement of the object relative to the objective lens is measured in consideration of the property that a shape of an detected image (70) detected by the detection section varies depending on the displacement of the object relative to the objective lens.