In an optical module having a silicon substrate, a plurality of optical semiconductor devices and optical waveguides for performing transmission of optical signals by the semiconductor devices integrated on the silicon substrate, the silicon substrate is doped with an impurity to increase the number of carriers in the silicon substrate for suppressing optical crosstalk between the plurality of optical semiconductor devices, the optical waveguide is composed of a core part and a peripheral cladding layer of the core part, or optical fibers each coupled with each of the semiconductor devices, and an electrical resistivity of part or all of the silicon substrate is 0.1 .OMEGA.cm or less, or a lower part of an optical semiconductor light receiving device is made high in resistance, and a lower part of an optical semiconductor light emitting device is made low in resistance in which construction, each of the optical semiconductor devices is locally coated with a transparent resin, and all the parts protruding upward from the optical waveguide are coated with a light absorbent such as one or plural caps.