PURPOSE: To provide the optical switch which is capable of increasing an on/off ratio in switch parts, eliminates the pass loss of the signal light at the boundary between waveguides and the switch parts and is small in the absorption loss of the signal light generated in the process that the signal light is successively guided in the waveguides.

CONSTITUTION: Optical waveguide layers of the waveguides 2A to 2C are formed of multiple quantum well layers 31. The thickness of the parts 31' which are the switch parts 30a, 30b of these multiple quantum well layers 31 is set larger than the thickness of the other parts, by which the energy band gap thereof is made smaller than that of the other parts in a range where the energy band gap is not smaller than the energy of the signal light.