Abstract: A wavelength detecting device is used for a wavelength multiplexing optical communication system, is simple and inexpensive, and can strictly control the wavelengths of signal beams that are close to one another. The device has a path splitter and a wavelength selector. The path splitter splits a beam into at least three beams having different path angles. The wavelength selector filters the split beams into beams having predetermined wavelengths. One of the filtered beams is provided as a signal beam and the other beams are provided as monitor beams used to detect the wavelength of the signal beam.