A belt drive device includes a drive source, a drive pulley, a driven shaft, a driven pulley, a belt, and a speed detecting device. The drive pulley is rotated about an axis thereof by the drive source. The driven shaft is rotatable about an axis thereof. The driven pulley is secured to and in axial alignment with the driven shaft. The belt is annularly formed of a metallic flat belt and mounted around the drive pulley and the driven pulley. The speed detecting device includes an encoder disk and a single optical sensor. The encoder disk has an optical pattern formed thereon. The optical sensor reads the optical pattern. The speed detecting device detects a rotational speed of the driven shaft based on a result of the optical sensor having read the optical pattern. The encoder disk is secured to and in axial alignment with the driven pulley.