An input detection system includes a display device with a detection function that includes a plurality of detection electrodes arrayed in a display region, an electronic apparatus case including an accommodation portion accommodating the display device with the detection function and a cover covering the display region of the display device with the detection function, and an input device that is mounted on the cover of the electronic apparatus case so as to rotate around a rotating axis extending in a normal direction to a surface of the cover and includes a first electrode and a second electrode facing the detection electrodes.