The Mach-Zehnder type thermo-optic switch with antenna-coupled Y-junction was demonstrated for the first time. The selective photobleaching of the polysilane films are used for fabrication. The branching angle was 3 deg and the longitudinal length was around 30 mm, which was reduced as around 60 % from the conventional structure. The beam propagation method was used for optimizing the design and the calculated transmission loss was only 0.16 dB. Our experiments showed the ON/OFF ratio of 9.9 dB and the switching power of 11.4 mW.