An eccentric check butterfly valve is used as a control valve. So to speak, the valve can be used not only a butterfly valve to control the flow rate or the pressure, but also a check valve to prevent the reverse flow. In order to play these two roles at the same time, the rotating axis of a disk is located eccentrically from the central axis of the disk. As an optimum design, a characteristic function is defined for evaluating a stability of the flow field. The height of an eccentric axis is determined by the characteristic value, and a maximum resultant pressure of the valve disk is calculated by employing waterhammer analysis when a sudden pump trip-out occurs. Based on the result of the flow filed analysis and the waterhammer analysis, a structural optimization is progressed. Through these design processes, it is derived an optimum design, satisfying the structural safety and the stability of the flow field as well as being lighter than previous models.