An optical input/output device includes a phase modulator element and an optical element. The phase modulator element includes a plurality of pixels arranged in a matrix and is configured to change an optical phase of signal light by applying a driving signal corresponding to a phase pattern. The optical element is configured to convert a direction of exit of the signal light so as to irradiate each pixel with the signal light from the input port. A pattern generator unit includes superimposing means for superimposing a periodic phase pattern having a predetermined period in at least one direction in a plane of the phase modulator element, and means for controlling an amplitude of the periodic phase pattern. The signal light is diffracted to a position according to the period of the superimposed periodic phase pattern, so that light intensity of the signal light is dispersed.