PURPOSE: To make it possible to enhance the transmitting efficiency of drive power upon low speed rotation or start of rotation to enhance the performance of power transmission, by providing a vane mechanism in which magnetic members are provided to either one of vanes or a casing.

CONSTITUTION: A drive coupling device body 13 in a four wheel drive vehicle comprising a vane pump VP and a hydraulic circuit 21 inherent to the pump VP. A rotor 19 in the pump VP is coupled to a first rotary shaft 11 which transmits drive power to front wheels, and as well a cam ring 20a and an outlet side plate 20c which constitute a casing 20 are coupled to a second drive shaft which transmits drive power to rear wheels. Vanes 18 in the vane pump VP serving as a hydraulic pump, are formed of permanent magnets, and the rotor 19 is made of nonmagnetic materials. Further, the cam ring 20a is made of magnetic materials. Accordingly, the vane 18 are normally urged in their projecting directions so that the coupling function thereof are sufficiently maintained.