The present invention provides an optical module using optical waveguides which permits easy fabrication of a PLC type optical multiplexer/demultiplexer with reduced loss. The optical module comprises cores which are contiguous to each other through a dicing groove adapted to position and fix an optical functional part such as a wavelength selection filter and a core branched from the said cores. A slight gap is present between the former and the latter core and both cores are narrower in width in the vicinity of the dicing groove than the other portions.