To obtain an optical NOT logic circuit which facilitates size reduction and integration by using a symmetrical Mach-Zehnder type optical waveguide circuit as a logical operation circuit.

Light projected by a laser diode 23, which is driven by a laser diode drive circuit 25, is split into two by a symmetrical Mach-Zehnder type optical waveguide 27. They are put together again via optical waveguide branch circuits 33 and 35 and converted by a photodiode 29 into an electric signal, which is outputted from a photodiode output circuit 31. When a switch 41a of a modulating circuit 41 is turned on to apply DC voltages to modulating electrodes 37 and 39, a light output goes up to a high level and when the switch 41a is turned off to apply no DC voltage to the modulating electrodes 37 and 39, the light output goes down to a low level. This optical NOT logic circuit can be formed into an optical IC, by performing logical operation through the not of the symmetrical Mach-Zehnder optical waveguide circuit and can easily be made small-sized and integrated.