A reflective optical detecting device is for detecting movement of an object in a predetermined area. The reflective optical detecting device includes a light emitting unit, at least two optical detectors, and a comparing unit. The light emitting unit is used for illuminating the predetermined area. The optical detectors are disposed at opposite sides of the light emitting unit, and each of the optical detectors is associated with a respective detection region within the predetermined area for generating an output signal in response to light reflected from the object and detected thereby. The comparing unit is coupled to the optical detectors for receiving the output signal from each of the optical detectors, and is operable to output a comparison result according to the output signal of each of the optical detectors. The comparison result is to be used for determining the movement of the object.