To restrain deviation of the optical axes of a receiving light and a transmitting light, and improve and stabilize light receiving efficiency, by installing a probe light source, detecting deviation of optical axes of a probe light and a laser light (transmitting light) with a two-dimensional light detecting array, and adjusting the tip position of a relay optical system.

A probe light from a laser light source 29 is made a reverse propagation receiving light through an optical mixer 9, converted to a Gaussian beam almost equivalent to a pulse laser beam 21 by a light coupling optical system 27, ad made to enter a beam splitter 5. On the polarization isolation surface of the beam splitter 5, incident points of the pulse laser beam 21 and the probe light are made to coincide with each other. The respective reflected light and transmitted light are made to enter the two-dimensional light detector array 32, in the state that optical axes almost coincide with each other. The array 32 detects angle deviation and directions of the optical axes, and controls position adjusting mechanism 33 of an optical fiber in such a manner that the measured value becomes zero. Thereby, when the optical axes of the receiving light 24 and the pulse laser beam 21 change on account of temperature change, vibration, etc., the angle deviation can be always restrained.