To shorten the rising time of the light output to an impressed bias voltage by providing the modulator with plural quantum well layers in which the energy gaps of exciton are substantially equal to each other when the modulation voltage of a prescribed value is impressed on its optical waveguide layer.

The multiple quantum well optical waveguide layer 5a is constituted by laminating the well layers 13a to 13c having the compsns. or thicknesses different from each other via barrier layers 14. At this time, the compsns. and thicknesses of the well layers 13a to 13c are so set that the respective energy gaps of the well layers 13a to 13c are equaled to each other when the prescribed bias voltage is impressed on the multiple quantum well optical waveguide layer 5a via p-type electrodes and n-type electrodes. Then, light output hardly decreases when the bias voltage to be impressed is small. The light output decreases sharply when the bias voltage increases to the prescribed voltage. Consequently, the rising time of the electric field absorption type optical modulator may be shortened.