A plurality of valves are triggered so that a first valve is triggered by a first trigger signal and a second valve is triggered by a second trigger signal, the first trigger signal and the second trigger signal are each generated by generating a pulse width modulated frequency signal of the first frequency f1 and a dither signal of the second frequency f2, where f1>f2, the trigger signal is generated by modulating the clock pulse duty factor of the frequency signal with the dither signal, and the dither signals of the first trigger signal and of the second trigger signal are synchronized with one another at chronological intervals.