PURPOSE: To position a light pen by providing transparent waveguides with detectors on a display panel.

CONSTITUTION: A titanium oxide film is formed on a liquid crystal display panel by the high frequency sputtering method, and longitudinal waveguides 4 and transverse waveguides 5 are formed in a matrix by the photoetching technique. Linear image sensors 6 and 7 using amorphous silicon are formed on the longitudinal waveguides 4 and in the right face part of the transverse waveguides 5 respectively. Longitudinal and transverse photodetector arrays are successively scanned, and the light intensity is miximized when the detecting element corresponding to the waveguide, with which the light pen is in contact, is operated. Thus, the position of the light pen is clarified.