An arrayed waveguide grating (AWG) module monitors the quality of a wavelength division multiplexed (WDM) multi-channel optical signal without using a special measurer in a wavelength division multiplexing (WDM) system, and a device monitors a WDM optical signal using the AWG module. The AWG module includes an input waveguide, a first star coupler, an AWG unit, a second star coupler, and an optical power measurer. The input waveguide receives the WDM optical signal via an optical signal transmission medium. The first star coupler splits the power of the received WDM optical signal and transmits split optical signals to waveguides of the AWG unit. The AWG unit processes the split optical signals so that they have phase differences which are proportional to the difference between the lengths of the waveguides included in the AWG unit. The second star coupler focuses the optical signals received from the AWG unit at different locations by causing mutual interference between the received optical signals. The optical power measurer is directly connected to the second star coupler, and generates electrical signals depending on power values of the optical signals focused by the second star coupler. As a result, the characteristics of the optical signals of different channels constituting a WDM optical signal can be measured without a special measurer, and the spectrum of an optical signal with respect to the overall wavelength band can also be obtained. Also, the AWG module has a structure which is suitable for mass production.