A power transmission device includes: an axle housing having a central hole where a drive shaft is penetrated; a wheel hub disposed in an outer portion of the axle housing via wheel bearings; a speed-reducer housing chamber provided in the wheel hub to cover an opening end portion of the axle housing; a gear speed reducer, formed inside the speed-reducer housing chamber, to decelerate a rotation of the drive shaft and transmit the rotation to the wheel hub; and a wet-type brake mechanism housed in a brake chamber formed between the axle housing and the wheel hub, and actuated such that braking is applied to a rotation of the wheel hub relative to the axle housing. Further, floating seals are disposed between a lubrication space and the speed-reducer housing chamber, and a circulation passage for oil that includes an oil filter is connected to the lubrication space.