The tire condition of each tire of a vehicle is detected, which information is transmitted by means of electromagnetic waves from the tire condition acquisition devices to a monitoring device that receives the electromagnetic waves transmitted from tire condition acquisition devices via a receiving antenna, which monitoring device contains means for displaying the electric field intensity of the electromagnetic waves transmitted from the tire condition acquisition devices. Thus, the electric field intensity at a mounting position can easily be determined when mounting the receiving antenna, enabling easy discovery of the optimal location for mounting the receiving antenna.