A planar lightguide circuit with the optical intensity modulator array structure capable of generating less influence upon the adjacent channels, by inserting a block optical waveguide between optical channels formed in a planar lightguide circuit chip having the optical intensity modulator array. The circuit is provided with a plurality of channel arrays each including an input waveguide, a waveguide modulation region connected to the input waveguide, for modulating an input light wave, an output waveguide connected to the waveguide modulation region, for outputting the modulated light wave, and a modulating unit for modulating the light wave that is disposed in the vicinity of the waveguide modulation region. The circuit chip further includes at least one first channel having a first optical intensity modulator provided with said modulating means, at least one second channel having a second optical intensity modulator provided with the modulating unit, and at least one block waveguide arranged between the first and second channels, for blocking the mutual interference between the light waves in the adjacent channels.