PURPOSE: To suppress a side lobe or to eliminate a notch of a pattern and to obtain a radiation pattern of the antenna properly mounted on an automobile by devising the amplitude and the phase condition of each circularly polarized element antenna in an onvehicle planar antenna system.

CONSTITUTION: Power fed to a circularly polarized element antenna close to a feeding point in the on-vehicle planar antenna system is increased and power fed to a peripheral circularly polarized element antenna is gradually decreased to decrease a level of the side lobe. As a result, in the case of searching a radio wave through the scanning by an automatic tracking device, the possibility of the tracking of the side lobe to be a main lobe is precluded. Concretely the length of a feeding insert section 21 of a helical antenna 20 of the circularly polarized element antenna is set so that the radiation power of the helical antenna 20 apart from the feeding point is larger than the radiation power of the helical antenna 20 close to the feeding point.