The decelerating device according to the disclosure includes first and second planetary gear mechanisms arranged in an inner space of a hollow type electric motor having an annular rotor. The first planetary gear mechanism includes a first ring gear integral with the rotor, a non-rotatable first carrier for supporting a first pinion gear engaged with the first ring gear to be rotatable, and a first sun gear engaged with the first pinion gear. The second planetary gear mechanism includes a second ring gear integral with the rotor, a second carrier supporting a second pinion gear engaged with the second ring gear to be rotatable and connected to the output shaft, and a second sun gear engaged with the second pinion gear and connected to the first sun gear.