An object can be displayed on a screen of a two-dimensional coordinate system based on xyz-coordinate values of the object in a three-dimensional coordinate system, operation information of a two-dimensional coordinate system with respect to the object can be received from an input device, and whether the operation information is in accordance with a predetermined rule or not is determined. If the operation information is not in accordance with the predetermined rule, xy-coordinate values of the object can be updated in accordance with the operation information. If the operation information is in accordance with the predetermined rule, a z-coordinate value of the object can be updated in accordance with the operation information.