To provide a drive power transfer device which can constitute a mechanism for discharging air in the oil on a small scale and at low cost.

Since the through passages 155a3 to 155a5 of a stem bleeder 155a are provided to a piston body part 153a, the inner shape (structure) of a center cover 65 is prevented from becoming complicated, and a casting mold used for manufacturing the center cover 65 is prevented from becoming expensive. The manufacture of the casting mold is prevented from becoming difficult. Since the movement of the pin 155a6 inserted in the through passage 155a3 to the drive-power adjustment part 100a side is regulated by a bearing B3a, it is not necessary to separately attach a plate member or the like which is used for preventing the pin 155a6 from bursting out of the interior of the first through passage 155a3 to the drive-power adjustment part 100a side. The need for the space used for attaching the plate member can be eliminated. Thus, cost reduction and miniaturization of the whole drive-power adjustment mechanism 60a can be realized.