The present invention relates a pulse width filter generating a modulation signal that is increased in synchronization with one of an increasing edge and a decreasing edge of the input signal and is decreased in synchronization with the other of the increasing edge and the decreasing edge, and transmitting the input signal of the modulation signal. The input signal passed through the filter unit is inverted thereby being an output signal. The pulse width filter controls the increasing and the decreasing of the modulation signal according to the output signal and the input signal passed through the filter unit, and the modulation signal is a signal to determined whether the pulse width of the input signal is more than the predetermined cut-off pulse width.