PURPOSE: To attain input of a pattern shown by bright lines on a comparatively dark screen by using a CRT control part which cam display the multi-value brightness, a comparator which can decide the degree of brightness and a central control part.

CONSTITUTION: A comparator part 6 compares the specific brightness designated by a central control part 3 with the brightness sent via an interface part 5 and detected by a light pen 4. Thus the intensity of the light supplied from the pen 4 is decided. When this light intensity is equal to the level of the designated specific brightness, the part 6 transmits signals. The part 3 receives the signal from the part 6 and sends the signal to a CRT control part 2 in order to display the position where the signal is received from the part 6 with the 2nd brightness level. Then the part 6 sends the signal to the light of the 3rd brightness level to be designated. In this case, the signal which changes the display into a prescribed pattern is sent to the part 2.