1. A method for determining a lean angle of a body, comprising: a head-position obtaining step for obtaining a position of a head; a search region determination step for determining a plurality of search region spaced with an angle around the head; an energy function calculating step for calculating a value of an energy function for each search region; and a lean angle determining step for determining the lean angle of a search region with a largest or smallest value of the energy function as the lean angle of the body.