We have studied theoretically the effect of gravity on the rough solid-liquid interface and have shown that its tension is enhanced by gravity when gas is adsorbed on it. As a result, the apparent equilibrium contact angle, which has been considered not to be influenced by gravity so far, can be raised by gravity for rough surfaces. The calculated dependence of contact angles on gravity under the ordinary conditions of the sessile drop method is large enough to detect by experiment. The observed asymmetric deviations of the Wenzel contact angles caused by the adsorption of gas at the solid-liquid interface and caused by the adsorption of liquid at the solid-gas interface are explained in terms of this gravitational effect.