The present invention provides a touch panel input device that reliably prevents degradation of a piezoelectric substrate. During inactive periods in which a touch panel input device is not used, the connection between a set of output terminals of an AC power supply circuit and a set of associated drive electrodes on the piezoelectric substrate is switched with a selector switch so that the drive electrodes are shorted. During inactive periods, no DC potential is applied to the set of drive electrodes even if a charged conductive body comes into contact with the drive electrodes or a drive power source pattern connected thereto. This makes it possible to prevent the degradation of the piezoelectric substrate.