A speed reducing device includes a motor and a speed reducing mechanism. The motor includes a stator portion and a rotor portion. The rotor portion includes a first eccentric ring and a second eccentric ring. The speed reducing mechanism is partially accommodated within the rotor portion. The speed reducing mechanism includes a first roller assembly, a second roller assembly, a third roller assembly, a first cycloid disc set and a second cycloid disc set. The first roller assembly includes at least one first roller. The second roller assembly includes at least one second roller. The third roller assembly includes an output shaft and at least one third roller. The first cycloid disc set is mounted around the output shaft and disposed within the first eccentric ring. The second cycloid disc set is mounted around the output shaft and disposed within the second eccentric ring.