An apparatus for measuring the wavelength, optical power, and an optical signal-to-noise ratio (OSNR) of each optical signal in wavelength-division-multiplexing optical communication includes: elements for splitting a part of wavelength-division-multiplexed (WDM) signals, elements for amplifying the WDM signals and generating spontaneous emission light simultaneously, elements for reflecting a predetermined section of the spontaneous emission light and generating an optical reference signal, and elements for combining the optical reference signal with the part of the WDM signals split by the splitting elements and generating a combined light. The apparatus has components for filtering the combined light at a fixed temperature and generating a waveform which is the same as an optical spectrum of the combined light in the time domain. The apparatus includes elements for converting the waveform into an electrical signal and components for signal processing that measure the wavelength, the optical power, and the OSNR of the WDM signals.