To provide a waveguide input/output device which is used for an acousto-optic deflection element of a waveguide type and with which the intensity distribution of the exit light of the device attains a gauss distribution exactly in the progression direction of the guided light and in the direction perpendicular to this progression direction.

A thin-film waveguide 2 is provided thereon with a grating 7 for incident and a grating for exit. The height of the grating 7 for incident is constant in the progression direction of the guided light and increases gradually in the progression direction of surface acoustic waves, i.e., in the direction perpendicular to the progression direction of the guided light. The height of the grating for exit increases gradually in the progression direction of the guided light and is constant in the direction perpendicular to the progression direction of the guided light.