High pressure pulsation generated by vane pumps causes serious vibration or noise in oil pressure systems. As vibration or noise reduction by increasing the stiffness of the system is difficult, reduction by decreasing the pulsation is required. This paper presents an interferential method to decrease the pulsation and a simulation of the interference. The interferential device is composed of tandem vane pumps, dual pipelines and an interferential chamber, and is supplied with pulsation flows of phase difference 180°. The interference effect occurs in the upper stream of the chamber because of the flow speed of the pulsations ; both the pulsation and the phase difference decrease gradually. The simulation clarifies the effect seen in the experiment. We also indicate the similar effect in flows separated by an elastic film.