PURPOSE: To improve the efficiency of a space and the operability of a mechanical mouse by making it unnecessary to use a mouse pad for an optical mouse.

CONSTITUTION: Optical oscillators 1 to 4 are arranged on respective vertexes of a rectanglar area 7 so that the area 7 is fully covered with light beams projected from respective parts 1 to 4. The projected beams are photodetected by a receiving part 6 fixed to an operation part 5 to be optionally moved in the area 7, so that the intra-area position of the operation part 5 is calculated by a measuring operation part 9. An arrow to be a marker is displayed on the display screen of a CRT 13 based upon the calculated positional data. Thereby the use of a mouse pad is made unnecessary and a working space can be expanded. Since a ball is not used, the operability of the mouse can be improved.