A power transmission apparatus suitable for use with a four wheel drive apparatus of an automobile which can prevent spouting of working fluid through a sealing portion or the like and introduction of external air into the apparatus through a sealing portion. The apparatus comprises a hydraulic transmission coupling mechanism of the vane pump type connected to a pair of shafts to transmit driving force between the shafts and having a pump chamber formed therein. A tank is provided in an integral relationship with the coupling mechanism and accommodates working fluid in an enclosed condition therein. A volume variation absorbing mechanism is provided near an axis of rotation of the coupling mechanism for absorbing a variation in volume of the working fluid in the pump chamber.