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.