A thermo-optic switch using a small drive power while exhibiting a reduction in the coupling loss caused by the coupling to optical fibers and a switch speed of several hundred microseconds or less. A method for manufacturing the thermo-optic switch and a method for changing an optical line switching using the thermo-optic switch are also disclosed. The thermo-optic switch includes a substrate having etched portions at regions respectively corresponding to input and output terminals of the thermo-optic switch, a lower clad layer formed over the substrate, the lower clad layer having an input taper formed at the region corresponding to the input terminal and adapted to convert a circular mode, input from an optical fiber connected to the input terminal, into an oval mode having a rib shape, and an output taper formed at the region corresponding to the output terminal and adapted to convert the oval mode into a circular mode allowed to be input to an optical fiber connected to the output terminal, a core layer formed over the lower clad layer and provided with branched waveguides having a rib structure, the branched waveguides selectively receiving the oval mode from the input taper and outputting the received oval mode to the output taper, an upper clad layer formed over the core layer, and switching electrodes formed on the upper clad layer and selectively activated to apply heat to an associated one of the branched waveguides in such a fashion that an effective refractive index difference occurs between the branched waveguides, thereby causing the branched waveguides to selectively receive the oval mode from the input taper.