A lens array (38) is mounted on the front surface of an optical fiber array (32) holding optical fibers (35, 36) for input and emission. The lens array (38) includes: an input lens (40a) for converting a signal light (45) emitted from the optical fiber (35) into parallel light or converged light; and an output lens (40b) for converging the returned parallel light and connecting it to the optical fiber (36). A rectangular prism (34) having a form of a rectangular equilateral triangle is arranged in front of the optical fiber array (32) having the lens array (38). The signal light (45) emitted from the optical fiber (35) is totally reflected twice by the rectangular prism (34) and comes into the optical fiber (36). A transparent rectangular rotary block (33) is arranged between the lens array (38) and the rectangular prism (34). The outgoing signal light (45) and the returning signal light (45) have optical axis shifted by the rotary block (33) where the signal light passes. With this structure, it is possible to obtain a small-size variable light attenuator having a high control accuracy of the attenuation quantity of the signal light and a high resolution.