A roadside apparatus according to the first aspect includes: a housing including an antenna mounting surface; an antenna connection terminal being provided on the antenna mounting surface, and supporting both a rod antenna and a planar antenna; and a circuit being housed in the housing, and being configured to perform road-to-vehicle communication via a connection antenna connected to the antenna connection terminal. An area of the antenna mounting surface is equal to or larger than an area of the planar antenna. An antenna selected out of the planar antenna and the rod antenna is connected as the connection antenna to the antenna connection terminal.