To provide an auxiliary power device, requiring a not very large installing space, hardly having a rotation loss, and causing a not very large increase in vehicle weight.

This auxiliary power device has at least a hydraulic device 1 for converting rotational drive of a propeller shaft into a hydraulic pressure to accumulate it in an accumulator 201 and for converting the accumulated hydraulic pressure in the accumulator 201 into the rotational drive of the propeller shaft. Hydraulic oil is fed to a pump motor part 10 side from a feed port 14 at any time and is taken out of a taking-out port 15 to a hydraulic controlling part 20 side, so that a flow of the hydraulic oil does not change.