In order to distinguish between a deterioration in a received optical signal strength resulting from optical axis deviation, and a deterioration in the received optical signal strength due to the effects of dust and the like, this detecting system is provided with: a transmitter provided with a light emitting unit for transmitting a first optical signal and a second optical signal having mutually different wavelengths and divergence angles; and a receiver provided with a detecting unit for receiving the first and second optical signals and outputting a first reception signal indicating the reception strength of the first optical signal, and a second reception signal indicating the reception strength of the second optical signal, and an identification unit for determining a state of propagation of the first and second optical signals on the basis of an amount of variation in the first and second reception signals.