A light pen detection system used for a display system using a cathode ray tube (CRT) in which the operation for detecting the character display position on the CRT face is started when the operation switch of the light pen is turned on, and repeatedly progresses in synchronism with the picture repetition rate, and when a position marker is displayed at the character display position detected through such detecting operation, the operation switch of the light pen is turned off thereby to stop such detecting operation, whereby the character display position detected is obtained as the up-to-date one indicated by the light pen.