A mobile device comprising: a battery; a first communication means configured to be driven by the battery and to transmit a first transmission signal to a vehicle side; a second communication means configured to receive a certain electromagnetic field from the vehicle side, to be driven by an electromotive force induced by the certain electromagnetic field, and to transmit a second transmission signal in response to the certain electromagnetic field; a control unit that is configured to control the first communication means and the second communication means; and a circuit board on which the first communication means, the second communication means, and the control unit are mounted; wherein the first communication means includes a loop antenna that is formed by a conductive wire formed on the circuit board, the second communication means includes a transponder coil having a substantially rectangular shape, the transponder coil being disposed on the circuit board, one shorter edge of the rectangular transponder coil is placed near an edge of the circuit board such that the one shorter edge is directed towards the vehicle when a key plate of the mobile device is inserted into a keyhole of the vehicle, the key plate being attached to the second communication means, wherein the conductive wire is formed on the circuit board so as to enter a projected area of the transponder coil, the projected area being formed on the circuit board, from one longer edge of the projected area and exit the projected area from another longer edge of the projected area, wherein the conductive wire is formed on the circuit board so that the conductive wire passes through a central portion of the longer edge of the projected area of the transponder coil, and wherein the loop antenna is formed on the rear surface of the circuit board. a battery; a first communication means configured to be driven by the battery and to transmit a first transmission signal to a vehicle side; a second communication means configured to receive a certain electromagnetic field from the vehicle side, to be driven by an electromotive force induced by the certain electromagnetic field, and to transmit a second transmission signal in response to the certain electromagnetic field; a control unit that is configured to control the first communication means and the second communication means; and a circuit board on which the first communication means, the second communication means, and the control unit are mounted; wherein the first communication means includes a loop antenna that is formed by a conductive wire formed on the circuit board, the second communication means includes a transponder coil having a substantially rectangular shape, the transponder coil being disposed on the circuit board, one shorter edge of the rectangular transponder coil is placed near an edge of the circuit board such that the one shorter edge is directed towards the vehicle when a key plate of the mobile device is inserted into a keyhole of the vehicle, the key plate being attached to the second communication means, wherein the conductive wire is formed on the circuit board so as to enter a projected area of the transponder coil, the projected area being formed on the circuit board, from one longer edge of the projected area and exit the projected area from another longer edge of the projected area, wherein the conductive wire is formed on the circuit board so that the conductive wire passes through a central portion of the longer edge of the projected area of the transponder coil, and wherein the loop antenna is formed on the rear surface of the circuit board.