A vehicle-mounted antenna of the present invention includes a grounding conductor arranged on a vehicle interior side surface of a rear window that inclines at a predetermined inclination angle with respect to the horizontal plane, a slot and a U-shaped auxiliary slot formed in the grounding conductor, and an electrically conductive reflecting plate that protrudes in a direction that forms a predetermined angle with respect to the surface of the grounding conductor so as to slope downward towards the front of the vehicle from a location shifted downward in the vertical direction (front of the vehicle) from the slot on the surface of the grounding conductor at a location close to the slot.