On the vibrating mechanism of a valve which has an elastic plate installed parallel to the fluid flow, there are some monographs which dealt with musical instruments, such as clarinet and oboe, but the selfexcited vibration seemed to have been neglected. In the present paper it is described that when fluid passes through the narrow spaces, between the parallel plates which can move vertically by a spring action, an equation of motion of the valve and an equation of continuity which involves the compressibility of fluid are obtained. Then from them a differential equation of 3rd order is introduced and so by means of Routh's criterion, it is revealed that such a valve can give rise to a selfexcited vibration under a certain condition. An experiment verified this result by flowing air through this valve.