An input device comprising: a selection circuit connected to a plurality of electrodes provided on a capacitive touch panel and arranged to selectively connect the plurality of electrodes; a calibration resistor (R CAL) connectable to the selection circuit; a constant current source for supplying constant current to the plurality of electrodes and the calibration resistor (R CAL); an A/D converter connectable to the selection circuit to measure capacitance of the plurality of electrodes and a measured value measured when the calibration resistor (R CAL)is connected; and a control unit arranged to control the selection circuit for the A/D converter to measure the capacitance of one of the plurality of electrodes and the said measured value measured when the calibration resistor (R CAL) is connected, and arranged to determine a contact coordinate of a touch object on the touch panel by calculating a ratio of the capacitance to the measured value, wherein the said measured value measured when the calibration resistor (R CAL) is connected, is measured with all of the plurality of electrodes disconnected, and the calibration resistor (R CAL) is connected in parallel with the plurality of electrodes to the selection circuit; and, wherein the control unit is arranged to control the selection circuit to disconnect the plurality of electrodes except the one electrode so that the A/D converter measures a first voltage of the one of the plurality of electrodes, and then to ground the plurality of electrodes except the one electrode so that the A/D converter measures a second voltage of the one of the plurality of electrodes, and arranged to compare the first and second voltages to determine whether or not a noise influence is present on a detection surface of the touch panel. a selection circuit connected to a plurality of electrodes provided on a capacitive touch panel and arranged to selectively connect the plurality of electrodes; a calibration resistor (R CAL) connectable to the selection circuit; a constant current source for supplying constant current to the plurality of electrodes and the calibration resistor (R CAL); an A/D converter connectable to the selection circuit to measure capacitance of the plurality of electrodes and a measured value measured when the calibration resistor (R CAL)is connected; and a control unit arranged to control the selection circuit for the A/D converter to measure the capacitance of one of the plurality of electrodes and the said measured value measured when the calibration resistor (R CAL) is connected, and arranged to determine a contact coordinate of a touch object on the touch panel by calculating a ratio of the capacitance to the measured value, wherein the said measured value measured when the calibration resistor (R CAL) is connected, is measured with all of the plurality of electrodes disconnected, and the calibration resistor (R CAL) is connected in parallel with the plurality of electrodes to the selection circuit; and, wherein the control unit is arranged to control the selection circuit to disconnect the plurality of electrodes except the one electrode so that the A/D converter measures a first voltage of the one of the plurality of electrodes, and then to ground the plurality of electrodes except the one electrode so that the A/D converter measures a second voltage of the one of the plurality of electrodes, and arranged to compare the first and second voltages to determine whether or not a noise influence is present on a detection surface of the touch panel.