A multiwavelength simultaneous monitoring circuit capable of precise discrimination of wavelengths of a WDM (Wavelength Division Multiplexed) signal including multiplexed wavelength waves, and suitable for optical integrated circuits having large resistance to vibration. It includes a reference optical source, an AWG (Arrayed-waveguide grating) having periodic center transmission wavelength (or crossover wavelengths) corresponding to the wavelength spacing between multiplexed waves of the WDM signal, a first wavelength error detector for generating a wavelength error signal of an RF signal (reference optical signal) with respect to a zero-cross wavelength on the basis of a ratio between the levels of the RF signal outputted from two output waveguides of the AWG, a control circuit for locking the zero-cross wavelength to the wavelength of the RF signal in response to the wavelength error signal, and a second wavelength error detector for generating wavelength error signals of the multiplexed waves of the WDM signal by detecting signal levels of respective wavelength signals of the WDM signal outputted from each pair of output ports of the AWG.