A reception strength detection unit is connected to reception antennas located in a vehicle compartment and detects a reception strength of a radio wave from a mobile terminal for each of the reception antennas. A reception determination unit is connected to an auxiliary antenna located at a predetermined position that is a blind spot of the reception antenna and determines whether the radio waves has been received by the auxiliary antenna. When the radio wave has not been received by the auxiliary antenna, an existence position calculation unit calculates an existence position of the mobile terminal based on the reception strength detected by the multiple reception antennas. When the radio wave is received by the auxiliary antenna, the existence position determination unit determines the predetermined position at which the auxiliary antenna receiving the radio wave is located as the existence position of the mobile terminal.