Abstract: A optical isolator apparatus suppressing transmission of light in a backward direction having transmitting light detecting stages includes optical fibers respectively arranged at input and output stages without using optical distributers requiring a specifically-processed coating. The apparatus can be used for detecting transmitting light. In other words, without using optical taps requiring a specifically-processed coating. The apparatus includes a first detecting optical fiber adapted to receive multi-reflected light generated in the interior of the optical isolator apparatus and to detect input-stage detection light based on the multi-reflected light and, a first lens adapted to collimate the input signal light. An output optical fiber is adapted to transmit an output signal light. A second detecting optical fiber is adapted to detect backward light reflected from the output optical fiber and introduced again in the interior of the optical isolator apparatus.