PURPOSE: To very sensitively measure a long distance measuring range in a small move quantity of a movable mirror, by using a plurality of light delay loops, optical-path lengths per circuit of which vary with one another as a means for constituting reference light from light having a plurality of different optical-path lengths.

CONSTITUTION: Light emitted from an incoherent light source 100 is divided into a signal light and reference light by a half mirror 101-1. The signal light is led to an object 104 to be measured, and reflected light generated on the surface and/or in the inside of the object 104 reaches to a light detector 105 by a half mirror 101-2. First, the reference light is led to a first delay means 103-1. In the inside of the means 103-1, the incident reference light is led to an optical fiber loop 107-1 by an optocoupler 106-1 for coupling a part of the incident light at a proper coupling ratio. This light passes through a light frequency shifter 110-1 and a light amplifier 120-1, and at every circuit, a part thereof is taken out of the loop 107-1 by the optocoupler 106-1.