an apparatus for monitoring a wavelength division multiplexed optical signal transmitted onto a main optical path includes : an amplified spontaneous light emitter for generating an amplified spontaneous emission signal and transmitting the same to onto a reference optical path ; at least one fiber bragg grating for sending out an amplified spontaneous emission signal having a reference wavelength by reflecting or absorbing only that part of the amplified spontaneous emission signal which consists of a reflection wavelength or an absorbance wavelength ; an optical switch for outputting at least one of the wavelength division multiplexed optical signals branched from the main optical path or one of the amplified spontaneous emissions including the reference wavelengths in response to a switching control signal input ; a tunable optical filter for filtering the optical signal outputted from the optical switch using varied filter characteristics , and for outputting the filtered optical signal ; and a control unit for measuring an optical wavelength , an optical power and an optical signal to noise ratio of the wavelength division multiplexed optical signal based on the analyzed reference wavelength .