PURPOSE: To provide an integrated optical circuit like a waveguide electrooptic optical switch which is superior in electrooptic stability and easily constitutes the multichannel output.

CONSTITUTION: Two parallel channel optical waveguides 3 and 4 forming a spontaneous polarization inversion periodic structure along the surface of a C plate lithium tantalate crystal 1 are formed on the surface of the crystal 1. A plane waveguide 5 is formed between them by proton transformation. Electrodes 6A and 7A to apply an electric field to optical waveguides are provided on channel waveguides, and electrodes 6B and 7B are provided near them. An optical buffer layer 11 which prevents electrodes from absorbing light is provided between the crystal face and electrodes.