Vibration amplitudes of a control valve in a cavitating flow were measured varying opening rate from 20% to 40%. Moreover, pressure on the valve sheet was measured synchronously to examine the correlation between the fluctuating fluid force and the response of the valve. As a result, the amplitude in the drag direction was increased drastically when the cavitation coefficient K_m exceeded 0.8. It was expected that this phenomenon was the coupling problem with cavitating flow and a structure from the correlation between the fluid force and the response.