Recently, images showing three-dimensional objects are generated because of development of computer graphics techniques. The screens of various electronic devices such as personal computers, mobile phones, and TVs use a two-dimensional coordinate system; therefore, the coordinate values of the objects of a three-dimensional coordinate system are displayed on the screens after being converted to those of the two-dimensional coordinate system. In order to carry out operations of the three-dimensional coordinate system with respect to the three-dimensional objects which are two-dimensionally shown in this manner, for example, a special mouse described in the art is capable of handling the operations of the three-dimensional coordinate system.