The purpose of this investigation is to develop the lifting surface theory of horizontal-axis wind turbines and devise the numerical method to solve the theory. The velocity potential is derived by the vortex theory. The integral equation that relates an upwash distribution to an unknown lift distribution is the same as that of a screw propeller. With the boundary condition pertinent to a wind turbine, the integral equation is solved by the method of Hanaoka. Furthermore, the pressure distributions of two kinds of rotor are measured. One is designed by means of the design method based on the lifting line theory, the other has rectangular blades of expanded shape with a constant geometric pitch. The pressure distributions on the blades of various working conditions are calculated using the concept of equivalent two-dimensional wing, and compared with the experimental ones. It is concluded that the presented lifting surface theory and the numerical method are applicable to wind turbines.