Optical switch

An optical switch includes at least one input optical semiconductor waveguide (5); two output optical semiconductor waveguides (3a , 3b) connected at a branch point to the input optical waveguide and diverging from the branch point (2) with a preset angle ϑ (degree) therebetween; and a refractive index controlling section (4) disposed in position on at least one of the output optical waveguides (3a) and apart from the branch point (2), for electrically causing the refractive index of the output optical waveguideto be reduced. A light attenuating layer for preventing radiation mode light leaked from a portion near the branch point to the outside of the optical waveguide from being re-combined with guided mode light in the output optical waveguide is disposed between the two output optical waveguides. The refractive index controlling section (4) is formed so that a length from the physical branch point between said two output optical waveguides to an output end portion of the refractive index controlling section may be set equal to or larger than 100 x ϑ/cos (ϑ/2) µm. A distance between the nearest portions of the refractive controlling sections of the output optical waveguides is set to be equal to or less than twice a spot size so as to improve the extinction ratio and excessive loss.