To obtain a waveguide type optical device which can attenuate only a component of TE mode of light propagated in a waveguide.

On a waveguide substrate 12 of the waveguide type optical device 21, a couple of waveguides 13C and 13D branching off from the waveguide 13 are arranged, and above them, electrodes 15 and 16 are formed to constitute a Mach-Zehnder type interferometer. An input-side optical fiber 17 is optically coupled with one end surface of the waveguide substrate 12, an Si buffer layer 22 is locally arranged nearby the other end surface, and an output-side optical fiber 18 is optically coupled with the end surface on this side. The Si buffer layer 22 is 95nm thick and an SiO2 film below it is 0.2μm. This part acts to attenuate the TE component.