To provide a power assist device which causes motorized devices with motors storing the kinetic energy of weights as motive power, like self- winders, to vary momentum to move weights, and which can efficiently cause the motorized devices to vary momentum.

The power assist device includes a plurality of mounting parts 5 for mounting the motor-equipped devices 6, and drive means for driving the plurality of mounting parts 5. The drive means cause the plurality of mounting parts 5 to rotate about a common axis and move up and down. In particular, the axis of rotation of the mounting parts 5 is not vertical, so the mounting parts 5 rotate also about their own axes. The drive means cause the plurality of mounting parts 5 to rotate about the common axis as well as their own axes. The drive means use planetary gear mechanisms 31 and 32.