Abstract: A displacement detecting apparatus for detecting a displacement between an object lens thereof and a measured surface in a non-contact manner. The displacement detecting apparatus comprises an object lens for focusing laser beam on the measured surface. A detecting optical system is provided to detect a focus error signal which represents a difference between a distance of the object lens from the measured surface and a focal distance of the object lens, in accordance with a reflected light from the measured surface. An actuator is provided to move the object lens in an optical axis in a manner to cause the object lens distance to coincide with the focal distance, in accordance with an output signal from the detecting optical system. A moving body includes a linear scale or a scale head and movable with the object lens. The linear scale is disposed coaxially with the optical axis and connected to the actuator. A support member is provided to movably support the moving body.