PURPOSE: To compensate a light pen position detecting error due to the characteristic change of the light pen or its signal system by allowing the light pen to touch upon the specific position of a display screen only once.

CONSTITUTION: If the light pen is allowed to touch upon the display position of a specific pattern PAT, a light pen output pulse signal WP is generated when scanning light passes through the light pen position. Consequently, a vertical positional signal LA and a horizontal positional signal CA are latched by latch circuits 1, 2 respectively. Since the positional signal LA latched by the latch circuit 1 corresponds to the vertical position of the specific pattern PAT, a detecting signal Ly is outputted from a detecting circuit 5. An arithmetic circuit 7 calculates the difference between signals CA and CAX at the timing generating the detecting signal Ly and the difference value is stored in an internal register. The difference value (x) is inputted to an arithmetic circuit 8, the horizontal positional signal CA obtained when the light pen is allowed to touch upon an optional position is inputted to the other terminal of the arithmetic circuit 8 and the difference signal is outputted to be compensated.