A touch key detects that an object touches the touch key. The touch key includes an electrode arranged to be touched with the object, an oscillator for outputting a high-frequency voltage and applies the high-frequency voltage to the electrode, a rectifier for rectifying and smoothing the high-frequency voltage output from the oscillator as to output a direct-current (DC) voltage, a reference voltage generator for generating a reference voltage, a subtracter for subtracting the reference voltage from the DC voltage output from the rectifier, a judging unit for detecting a voltage difference between the voltage output from the subtracter before and after changing and for determining that the object touches the electrode when the voltage difference exceeds a predetermined value. This touch key detects that an object touches the electrode with high sensitivity, and enhances the reliability of the judging unit.