A conventional optical signal processing device had a disadvantage where the temperature dependency of the spectroscopic characteristics of a spectroscopic element causes a deteriorated performance. In order to solve the temperature dependency, there has been a method to form a plurality of grooves for dividing a core on the array waveguide of the AWG. However, this method cannot avoid an excess loss and causes a high manufacture cost. By directly controlling the modulation characteristic profile formed by an element device of a spatial light modulator, athermalization can be achieved in a simpler and low-cost manner. This consequently provides a remarkable reduction of the light coupling loss in the spatial optical system of the optical signal processing device. More accurate temperature compensation can be realized that copes with an actual behavior of the device to a temperature fluctuation, including causing factors of a complicated temperature fluctuation of the optical system.