In a servomechanism operated by the PWM mode, the dither in the output becomes unstable at a certain value of the loop gain. When a low-pass filter is used as the feedback circuit, it cuts off the feedback of the dither component and stabilizes the dither. However, since the servo loop tends to become unstable on account of the lag characteristics of the compensating element, we must consider the stability of the whole loop as well as that of the dither in order to determine the stability limit. In this paper, the stability problems are studied for the case where the feedback circuit consists of a first order system. The optimum time constant of the system is also given in accordance with the pulse transmission lag in the forward loop. The analytical results are verified by the experiments carried out with an electrohydraulic servomechanism.