Provided is a planar optical waveguide circuit type variable attenuator capable of light attenuation by an arbitrary value in a range from 0 to 10 dB and also by not less than 30 dB, which is small-sized and is of power saving type. The attenuator is composed of a Mach-Zehnder interferometer circuit 30 having input waveguides 1a, 1b and an output waveguide 1d which are formed on an optical waveguide layer 3, and comprises a slit 12 formed on the waveguide layer in a longitudinally intermediate part of at least one of the optical waveguides 1a, 1b, 1d and in a direction crossing the optical waveguides 1a, 1b, 1d, and matching medium displacing means 16a, 16b for displacing index-matching medium 13 in a part of the slit 12 from a position including a path of propagation light in the optical waveguides 1a, 1b, 1d, and a position retracted from the path of propagation light.