A mobile device for a vehicle includes a receiving unit that receives transmission signals transmitted from individual transmitting antennas of a vehicle-side device; a control unit that determines distances to the individual transmitting antennas using the signal strengths of the received transmission signals; and a transmitting unit that transmits, to the vehicle-side device using wireless communication, data items indicating the distances. In a time period in which communication is not performed between the mobile device and the vehicle-side device, the signal strength of a received signal is measured, thereby obtaining a measurement value, and the measurement value is stored as a noise element in a memory. When communication between the mobile device and the vehicle-side device is performed, correction for noise is performed on the signal strengths of the transmission signals transmitted from the transmitting antennas, and the distances are determined.