In the present invention, a signal output unit (4) outputs a pulse signal having a signal level that changes in accordance with the rotation of a knob (3). A control unit (5) detects the signal level of the pulse signal in prescribed cycles to acquire detection signals. The control unit (5) detects the pulse width of each detection signal and determines whether the rotation speed of the knob (3) is increasing on the basis of the change over time of the pulse width. If the pulse width of the detection signal acquired after a determination has been made that the rotation speed of the knob (3) is increasing is determined to be greater than or equal to a first threshold and less than a second threshold, the control unit (5) determines that at least two undetected changes in the signal level of the pulse signal are included in the period corresponding to the pulse width.