Abstract: A waveguide type optical device with a traveling wave electrode structure, which has: a crystal substrate with electro-optic effect; an optical waveguide that is formed on the crystal substrate and waveguides light wave; a buffer layer that is formed on at least the optical waveguide and is of a dielectric material; and a signal electrode and an earth electrode that are formed on the buffer layer and control the optical output intensity of light wave waveguided through the optical waveguide. The traveling wave electrode structure has one signal electrode and two earth electrodes that are disposed sandwiching the signal electrode or one earth electrode that is disposed pairing with the signal electrode, at least the signal electrode of the traveling wave electrode structure is of a material that has a specific resistivity of 2.1×10−6 &OHgr;cm or lower at an environmental temperature that the waveguide type optical device is used.