PURPOSE: To simplify an apparatus by inputting coherent light beams from the ends of two optical fiber coils which are arranged so as to receive gravity waves in the different directions into the other ends, and measuring inference fringes of the coherent light beams.

CONSTITUTION: Laser light rays are used as coherent light rays 1. Two optical fiber coils A and B are formed by winding optical fibers around cylinders each having a diameter of, e.g. about 1m, by one million times. It is not necessary that the two coils A and B have the equal length and equal diameter. The coils A and B are fixed to the surfaces having an angle which is not zero, e.g. at a right angle. The light emitted from one hyperstable light source 1 is split into two beams. The light beams are inputted into the ends of the respective coils A and B. The light beams which are emitted from the other ends of the coils A and B are combined again. The light beams are made to interfere and inputted into an interferometer 2. By this constitution, an optical element and an optical apparatus can be formed with the coherent light source such as the laser light source, the optical fiber coils and the optical interferometer. Thus the apparatus can be made simple.