An input pen assembly is provided which selectively cooperates with the screen of a cathode ray tube of a CRT display and the tablet of a tablet input device. The assembly automatically activates either one of the CRT display and the tablet input device in response to a change in the magnitude of a pressure applied by the pen against the screen of the CRT or the tablet. The input pen assembly includes an electrode which has its one end projecting forwardly of the front end of a casing and which is used for detecting a signal from the tablet. The electrode is formed with a through-opening which defines a light transmission path for a signal from the CRT display. Additionally the input pen assembly includes a spring which resiliently biases the electrode so that its front end projects forwardly of the front end of the casing, and a movable member which is disposed for movement with the electrode when the latter is retracted against the resilience of the spring. The movable member is movable between a first position corresponding to an initial position of the electrode where no pressure is applied thereto, a second position corresponding to a position assumed by the electrode when a given pressure is applied thereto as it is brought into abutment against the tablet, and a third position corresponding to a position assumed by the electrode as an increased pressure is applied thereto when it is brought into abutment against the screen of a cathode ray tube. Two of the three positions are associated with position detectors for detecting the extant position of the movable member. A logical combination of output signals from these position detectors provides a determination as to the position assumed by the electrode, thereby selectively activating the tablet input device or the CRT display.