Abstract: An optical sensing system including a sensing plate, a light guiding device, and a detecting device is provided. The sensing plate has a light-pervious area. The light guiding device is disposed behind the sensing plate and used for reflecting an incident light proceeding along an incident direction, so as to form a first reflective light proceeding along a first direction and toward the light-pervious area. The incident direction is approximately perpendicular to the first direction. The detecting device is disposed behind the light guiding device. When an external surface of the light-pervious area is shaded by an object, the first reflective light is reflected by the object and forms a second reflective light proceeding toward the detecting device. Once receiving the second reflective light, the detecting device generates a sensing signal.