There has hitherto been known a power transfer device that includes a counter shaft disposed in parallel with an input shaft of a speed change mechanism, a differential device (differential gear) that has a ring gear (differential ring gear) disposed below the counter shaft and meshed with an output gear of the counter shaft, a case member that houses the speed change mechanism and the differential device, and a differential partitioning member that partitions a space in the case member into a differential chamber, in which the differential device is housed, and a storage chamber, in which oil (working oil) is stored (see Patent Document 1, for example). In the power transfer device, the differential partitioning member is composed of a part of the case member, a rib member provided to extend along the outer peripheral surface of the ring gear from the case member, and a hemispherical reservoir plate disposed so as to cover the differential device from the side opposite to the case member and tightly contact the inner peripheral surface of the rib member. Consequently, in the power transfer device, an inflow of oil from the storage chamber into the differential chamber is suppressed by the differential partitioning member.