PURPOSE: To easily provide a optical directional coupler having a prescribed wavelength characteristic.

CONSTITUTION: An under clad layer 6 and a core layer 1 are provided in superposition on the front surface of a silicon substrate 5. The core layer part of a region P constituted of inside walls 11V.10-1V.12V of a first optical waveguide and inside walls 20-1V.22V of a second optical waveguide of the first optical waveguide is removed by etching and an overclad layer 7-1 is provided on the front surface of the region P and the non-removed core layer 1-1. The overclad layer part of the region Q constituted of the region outer than the outside walls 11U.10-1U.12U of the first optical waveguide and the region outer than the outside walls 20-1U.22U of the second optical waveguide 20 and the non-removed core layer part are removed by etching, by which the first and second optical waveguides 10, 20 are provided; thereafter, an overclad layer 7 is provided over the entire surface of the regions Q, P and the first and second optical waveguides 10, 20.