1. A shoulder-line detecting method, comprising: receiving an image of a target person; defining a body space area from the received image; defining an external space area from the received image based on the body space area; extracting an edge area from a boundary area between the body space area and the external space area as the edge area; and calculating an energy value of the edge area while rotating an edge branched out from the edge area, based on the contact point of the branched edge and the edge area, and acquiring shoulder-line information from the branched edge when the energy value of the edge area is maximized, wherein the energy value of the edge area is maximized by determining an angle between the edge area and the branched edge that maximizes a ratio of the body space area to the external space area.