In a load weighing method for a wheel loader, an inclined angle of a ground surface on a slope where the wheel loader is working is detected. A change in a boom angle and a change in a pressure of a boom cylinder are detected while pivoting the boom and an attachment connected to the boom when a load is placed on the attachment. Weight of the load is calculated using the inclined angle of the ground surface, the change in the boom angle, and the change in the pressure of the boom cylinder as variables of a rotational dynamic equation for the pivot of the boom.