To provide a modulator that has a simple structure using no logic gate and a delay cell and processes a differential signal with a very high frequency.

The pulse width modulator is provided with 1st and 2nd transistors(TRs) T1, T2 whose conduction is controlled by an input signal and acting like a differential pair, a capacitive element C having 1st and 2nd terminals respectively connected to the 1st and 2nd TRs, adjustment means LC1, LC2, UC1, UC2 that adjust one level of the terminals and a comparison means CMP that is configured to supply an output signal the instantaneous value of which represents a sign of terminal voltage of the capacitive element. This invention attains high-speed and flexible control of a pulse width of the output signal Vout through a simple and analog circuit structure.