Provided is a thermo-optical phase shifter including a composite body having an optical waveguide formed by a core and a clad and having a ratio Δ of a difference between a core refractivity and a clad refractivity against the core refractivity which is 4% or above and a heater attached to the optical waveguide. The composite body has: a bridge structure portion arranged along a substrate surface and separate from the substrate surface via a void; and a fixed portion which supports the bridge structure portion with respect to the substrate and is continuous to the both ends of the bridge structure portion. The bridge structure has a half-circle-arc shape in the plane along the surface of the substrate.