A proximity sensing device includes: a light source, a sensing unit, a light guide unit, and a window. The light source emits light, which is guided by the light guide unit to the window. The emitted light reflected by an object is received by the same window. The light guide unit includes a partial-transmissive-partial-reflective (PTPR) optical element, whereby the light emitted from the light source is reflected by the PTPR optical element, while the light reflected by the object passes through the PTPR optical element. There is only one window required.