PURPOSE: To issue an abnormality warning when wavelength deviation between a transmission wavelength and a wavelength giving the minimum loss of a branching filter becomes more than an allowance value and to previously prevent the deterioration of reception sensitivity owing to wavelength deviation.

CONSTITUTION: The branching filter 21 branching wavelength multiplex signal light transmitted through an optical fiber for the respective wavelengths, receivers 221-22N receiving the output light of the branching filter 21 and a wavelength detection part 23 detecting wavelength deviation between the wavelength giving the minimum loss of the branching filter 21 and the transmission wavelength are provided. An alarm generator 24 judging abnormality when wavelength deviation detected by the wavelength detection part 23 becomes more than the allowable value and issues the alarm is provided.