PURPOSE: To make it possible to perform two-way conversation through only one piece of optical fiber, by using two optical power meter, and switching light-path switching mechanism.

CONSTITUTION: In an optical power meter, the following parts are provided: a photodetector 1; lenses 2 and 3, into which light from the outside is inputted, and which form a light receiving optical system; a light emitting element 4; and lenses 5 and 6, which emit the light to the outside and form an output optical system. Furthermore, a light-path switching mechanism comprising mirrors 7 and 8, which are rotated and driven, is provided. Before two-way communication is performed between both ends of an optical fiber cable by using two optical power meters, both ends of a piece of the optical fiber 9 are connected to the lenses 3 on the sides of the photodetectors. When a light signal, which is modulated with a speech signal, is sent from the side A to the side B, the switch of the optical power meter on the side A is depressed, and the mirrors 7 and 8 are rotated as shown by solid lines on the side A. Then, the light path is switched. The light from the light emitting element 4 is inputted into one end of the optical fiber through the lens 5, the mirror 8, the mirror 7 and the lens 3.