PURPOSE: To easily measure the displacement and speed of an object to be measured with high accuracy by offsetting errors caused when a polarizing plane maintaining optical fiber is given bending, impact, or vibration, by simultaneously transmitting measuring light rays and reference light rays through the optical fiber.

CONSTITUTION: First and second laser beams having different angles of polarizing planes and frequencies are propagated through a polarizing plane-maintaining optical fiber 12 while maintaining the relation between the polarizing planes after being emitted from a light source 10, and separated from each other through a beam splitter 36 in a measuring head 14, and the first laser beam is projected on an object 15 to be measured. The second laser beam is reflected by a reference reflection mirror 38, reversely propagated through the optical fiber 12 together with reflected light from the object 15, and transmitted to a measuring means 16 after being reflected by a half mirror 26. The measuring means 16 only transduces the component which is polarized in a prescribed direction through a polarizing filter 40 into electric signals. The electric signals are processed 44 by an optical heterodyne method and the speed and displacement of the object 15 are measured.