The waveguide-type optical device comprises a optical transmission element 12, an optical waveguide 20 optically connected to the optical transmission element, and a slab optical waveguide 30 formed of an electrooptic material and optically connected to the optical waveguide 20. In the waveguide-type optical device, a mode field width of the optical waveguide 20 has a value which is between a mode field width of the optical transmission element and a mode field width of the slab optical waveguide, whereby the connection loss between the optical transmission element 12, such as an optical fiber, etc. and the slab optical waveguide 30 can be small.