The purpose of this research is to study the free surface effects on the side force and the yaw moment acting on a ship hull advancing with a steady drift angle. First, on the assumptions of thin ship, small drift angle and low wave height, the problem was divided into a symmetric and an antisymmetric ones. Since the symmetric term had no effects on this problem, for the antisymmetric problem, approximate formulas to estimate the free surface effects were obtained on the assumption of slenderness. Next, by means of model experiment, free surface effects were examined, and it was ascertained that the above approximate formulas could explain the experiment results on the whole.