The present invention is configured so that an absorptive resin (31) for absorbing water is provided in a mesh-like shape on the surface of a window (6) covering a transmission and reception unit of an optical antenna (5). Providing the absorptive resin (31) in a mesh-like shape causes drops of water that have fallen on the surface of the window (6) during rain to be absorbed by the absorptive resin (31) before coalescing and reaching a size of a prescribed value or greater. As a result, reduction in heterodyne efficiency caused by large water droplets adhering to the surface of the window (6) can be prevented.
