An input signal processing device includes an oscillator circuit, a signal processing circuit, and a control circuit. The oscillator circuit performs an oscillation operation to output a clock signal. The signal processing circuit operates based on the clock signal outputted from the oscillator circuit. The signal processing circuit performs a predetermined process to an input signal when a level of the input signal changes and outputs a signal after the predetermined process. The control circuit instructs the oscillator circuit to start the oscillation operation based on a level change of the input signal. The control circuit instructs the oscillator circuit to stop the oscillation operation when the signal processing circuit finishes the predetermined process.