A vehicle-mounted antenna device is one in which rain water etc. do not accumulate around an antenna mast base structure and allowing a reduction of the antenna height. The vehicle-mounted antenna device having an antenna mast is constructed mainly from a base plate (10) on which a circuit board (11) is placed, a housing (20), and the antenna mast base structure (30) having a foldable structure. The housing (20) has a water discharge flow path (40) formed in a linear shape as viewed as viewed from the upper portion of the vehicle-mounted antenna device. The water discharge flow path (40) extends continuously underneath the antenna mast base structure while being inclined relative to the base plate. The water discharge flow path is constituted by, for example, the water discharge groove (41), which is formed at the bottom portion of the antenna mast receiving groove (22), and the water discharge flow path-cum-antenna mast receiving groove (42).