A mobile terminal position detecting device configured to detect an existence position of a mobile terminal in a vehicle compartment by detecting a radio wave from the mobile terminal, the mobile terminal position detecting device comprising: a reception strength detection unit connected to a plurality of reception antennas located in the vehicle compartment and configured to detect a reception strength of the radio wave from the mobile terminal for each of the plurality of reception antennas; a reception determination unit connected to an auxiliary antenna, the auxiliary antenna being connected to one of the plurality of reception antennas, the auxiliary antenna being located at a predetermined position that is a blind spot of the reception antenna to which it is connected in the vehicle compartment and configured to determine whether the radio wave from the mobile terminal has been received by the auxiliary antenna; an existence position calculation unit configured to calculate the existence position of the mobile terminal based on the reception strength detected by the plurality of reception antennas when the radio wave from the mobile terminal has not been received by the auxiliary antenna; and an existence position determination unit configured to determine, as the existence position of the mobile terminal, the predetermined position at which the auxiliary antenna receiving the radio wave is located when the radio wave from the mobile terminal has been received by the auxiliary antenna, characterized in that the reception determination unit is configured to determine that the radio wave from the mobile terminal has been received by the auxiliary antenna when the reception strength detected by the reception strength detection unit is larger than a predetermined threshold intensity, and whereby either (a) the auxiliary antenna is formed by wiring a linear member made of a conductive material in a plane shape and the reception antenna, to which the auxiliary antenna is connected, is connected to the reception strength detection unit through the linear member of the auxiliary antenna, or (b) the auxiliary antenna is a plate-like or foil-like member made of a conductive material, and the reception antenna, to which the auxiliary antenna is connected, is connected to the reception strength detection unit through the plate-like or foil-like member of the auxiliary antenna. a reception strength detection unit connected to a plurality of reception antennas located in the vehicle compartment and configured to detect a reception strength of the radio wave from the mobile terminal for each of the plurality of reception antennas; a reception determination unit connected to an auxiliary antenna, the auxiliary antenna being connected to one of the plurality of reception antennas, the auxiliary antenna being located at a predetermined position that is a blind spot of the reception antenna to which it is connected in the vehicle compartment and configured to determine whether the radio wave from the mobile terminal has been received by the auxiliary antenna; an existence position calculation unit configured to calculate the existence position of the mobile terminal based on the reception strength detected by the plurality of reception antennas when the radio wave from the mobile terminal has not been received by the auxiliary antenna; and an existence position determination unit configured to determine, as the existence position of the mobile terminal, the predetermined position at which the auxiliary antenna receiving the radio wave is located when the radio wave from the mobile terminal has been received by the auxiliary antenna, characterized in that the reception determination unit is configured to determine that the radio wave from the mobile terminal has been received by the auxiliary antenna when the reception strength detected by the reception strength detection unit is larger than a predetermined threshold intensity, and whereby either (a) the auxiliary antenna is formed by wiring a linear member made of a conductive material in a plane shape and the reception antenna, to which the auxiliary antenna is connected, is connected to the reception strength detection unit through the linear member of the auxiliary antenna, or (b) the auxiliary antenna is a plate-like or foil-like member made of a conductive material, and the reception antenna, to which the auxiliary antenna is connected, is connected to the reception strength detection unit through the plate-like or foil-like member of the auxiliary antenna. (a) the auxiliary antenna is formed by wiring a linear member made of a conductive material in a plane shape and the reception antenna, to which the auxiliary antenna is connected, is connected to the reception strength detection unit through the linear member of the auxiliary antenna, or (b) the auxiliary antenna is a plate-like or foil-like member made of a conductive material, and the reception antenna, to which the auxiliary antenna is connected, is connected to the reception strength detection unit through the plate-like or foil-like member of the auxiliary antenna.