A waveguide type variable optical attenuator is provided with a substrate
for forming a waveguide for optical signal propagation; a waveguide
element comprising 2 arm waveguides arranged on the surface of the
substrate for constituting a portion of the waveguide and cladding for
covering the arm waveguides and the surface of the substrate; and a
heater arranged on the surface of the waveguide element for heating the
arm waveguides. The 2 arm waveguides are connected thermally.