Patent ID: 7408650

Claim:
An optical-axis deflection type laser interferometer comprising: a reference ball serving as a measurement reference; a retroreflecting means disposed at an object to be measured; a laser interferometer length measuring apparatus that outputs a measurement value according to an increase or a decrease in distance between the apparatus and the retroreflecting means; and a rotational mechanism that rotates a beam projected from the laser interferometer length measuring apparatus around the reference ball; the optical-axis deflection type laser interferometer measuring a distance between the laser interferometer and the retroreflecting means that makes an optical axis of an outgoing beam projected from the laser interferometer length measuring apparatus mounted on the rotational mechanism and an optical axis of a return beam returned to the laser interferometer length measuring apparatus parallel to each other, based on center coordinates of the reference ball; wherein the improvement comprises: a displacement gauge that measures an error caused by a relative motion between the reference ball and the laser interferometer length measuring apparatus; and a rectilinear movement mechanism that displaces the displacement gauge and the laser interferometer length measuring apparatus in a direction of an optical axis of a measurement beam with respect to the reference ball while maintaining a relative positional relationship between the laser interferometer length measuring apparatus and the displacement gauge.