To provide a power transmission device capable of providing a high gear ratio, while being a compact constitution in a rotational axis direction, by using two planetary gear trains.

A first planetary gear train 10 and a second planetary gear train 20 are arranged on the same rotational axis X so that the rotational center of a first sun gear 11 and the rotational center of a second sun gear 21 coincide, and are arranged on the same diameter so that at least mutual parts of the first planetary gear train 10 and the second planetary gear train 20 overlap in the rotational axis direction. A first planetary carrier 13 and the second sun gear 21 are connected so that power can be transmitted, and the first sun gear 11 is connected to an input shaft 2, and a second planetary carrier 23 is connected to an output shaft 3.