A speed reducing device includes a motor and a speed reducing mechanism. The motor includes a stator portion, a shaft portion and rotator portion. The rotator portion includes first and second eccentric rings. The speed reducing mechanism includes first, second and third roller assemblies and first and second cycloid disc sets. These roller assemblies include first rollers, second rollers and third rollers. The first cycloid disc set is mounted around the first eccentric ring, and includes first teeth and second teeth. The second cycloid disc set is mounted around the second eccentric ring, and includes third teeth and fourth teeth. The first teeth are contacted with the first rollers. The third teeth are contacted with the second rollers. The second teeth and the fourth teeth are contacted with the third rollers.