One ends of a plurality of interface pins are attached to a substrate in a line. Optical semiconductor device and an electric circuit are mounted on this substrate. The other ends of the interface pins are fit into holes in an another substrate. Signals are exchanged between the two substrates via the interface pins. The interface pins are embedded in a dielectric material. A plurality of ground pins and/or ground through holes may be provided in the dielectric material around the interface pins. The dielectric constant of the dielectric material is less than the dielectric constants of the two substrates.