<p>[PROBLEMS] To provide a monitoring device of a simple structure capable of accurately acquiring light for monitoring and being reduced in size. [MEANS FOR SOLVING THE PROBLEMS] A lens array (17) is attached to an end face of an optical fiber array (12) holding optical fibers (14, 15) in parallel. A lens (19) is arranged in the lens array (17) so as to oppose to the end faces of the optical fibers (14, 15). A triangular prism (13) is arranged in front of the lens array (17). The incoming/outgoing surface (22) of the triangular prism (13) is inclined with respect to the lens array (17). A signal light L emitted from the optical fiber (14) is converted into parallel light by the lens (19) and comes into the triangular prism (13). The light is totally reflected by a reflection surface (20) and comes into a reflection surface (21). The incident angle of the signal light L coming into the reflection surface (21) is slightly smaller than the critical angle of the total reflection and a constant ratio kappa of the signal light L leaks outside from the reflection surface (21). By measuring the leak light La, it is possible to obtain the light quantity of the signal light L.</p>
