PURPOSE: To obtain an optical length changer of a simple structure where a high amplification factor is obtained, by combining one fixed plane mirror and rotary plane mirrors.

CONSTITUTION: The second plane mirror 2 is supported axially through a revolving shaft so that the second plane mirror 2 can be rotated around the intersection to the first plane mirror 1, and the third plane mirror 3 is so provided that the face vector is changed in the plane vertical to reflective faces of the fixed plane mirror 1 and the rotary plane mirror 2. The rotary plane mirror 3 is rotated around a revolving shaft 7 in accordance with the rotary plane mirror 2. If the rotation angle of the rotary plane mirror 3 is changed synchronously in such a way that the light is always made incident vertically to the m-th virtual image 5 formed by the fixed plane mirror 1 and the rotary plane mirror 2, an incident light 18 is reflected vertically by the virtual image 5 and goes back in the same optical path. When an angle θ1 formed by the fixed plane mirror 1 and the rotary plane mirror 2 is x° and the rotary plane mirror 2 is rotated at Δx°, the amplification factor is given by a formula.