A dust-proof sensing device includes a mechanical body, a feeding path, a first photoelectric sensor disposed above the feeding path, a second photoelectric sensor disposed under the feeding path, and an upper bracket. The mechanical body has a feeding path. The upper bracket is mounted above the feeding path. The upper bracket has an upper fastening portion fastened to the mechanical body, an upper wedging portion fastened at the upper fastening portion, an L-shaped upper light guiding holder fastened at the upper fastening portion, and a first light guider fastened at the upper light guiding holder. The upper fastening portion has a first inclined section. An inner edge of an upper surface of the first inclined section is intersected with a top edge of an inner surface of the upper wedging portion to form a clamping angle.