To mount an window-mounted on-vehicle antenna at a portion other than an windshield so as not to change antenna characteristics.

An antenna mounting apparatus 2 is provided for the purpose of mounting an window-mounted on-vehicle antenna 1 for communicating with a road side antenna. The antenna mounting apparatus 2 includes a portion 2 to be mounted which is formed from a synthetic resin and mounted in a portion, other than an windshield 4, inside a vehicle, and an antenna mounting plate 2C which is formed continuously to the portion 2 to be mounted. In this antenna mounting plate 2c, its rear side is formed as an antenna mounting surface 2g for mounting a radio wave transmission/reception plane 5b of the on-vehicle antenna 1 thereon.