The relation of the lifting force and lift of a circular disc valve which over the seat, is hereby analytically related. Making use of the momentum theory of fluid, the general formula of lifting force of any axially symmetrical valve which has a spontaneous profile is obtained. Then, taking viscosity into consideration, the relation of the lifting force and lift of a circular disc valve is developed. The value calculated from this formula is approximately in accordance with an experimental data obtained by Prof. Oki which has the following static characteristics : At first, in case of very small lift the lifting force of this valve is greater on account of viscosity. Next, when the lift is continually increased, it decreases suddenly to a minimum value and then turns to increase, but gradually reachs to a nearly constant valve.