occurring at some part of the recursive reflection section 104 can be compensated for with the remaining part of the widened area thus the detection precision can be improved although the cylindrical lenses are used in the light emission section for convenience in explanation the lenses are not limited by these lenses but various types of lenses may be used the focal point in this case means the position at which the lens forms the sharpest image in other words the focal point indicates the position at which the lens focuses the light at its peak in intensity fig8 shows an external view when the coordinate position input device 100 according to this invention is applied to a display board system 300 the display board system 300 has a large sized display unit e g a plasma display as shown in the figure and the coordinate position input device 100 of this invention is provided on the front surface of the display unit when the coordinate position input device 100 is applied in such a large sized display unit the detection precision of a coordinate input position is further improved as explained above according to one aspect of this invention the probe beam is substantially parallel.