Power transmission device for a four-wheel drive vehicle

A multiple plate clutch 11 is provided between a drive shaft 5 continuously extending to front wheels which are main wheels and a driven shaft continuously extending to rear wheels which are sub-drive wheels for distributing a part of the torque on the front wheels to the rear wheels. A tow-way clutch mechanism in which the drive shaft 5 is connected to a single hydraulic pump 26 via a clutch outer 17 and a sleeve 31 locks when the rotation speed of the drive shaft 5 exceeds the rotation speed of the driven shaft 6 beyond a predetermined rate to thereby drive the hydraulic pump 26 so as to put the vehicle in a four-wheel drive condition, irrespective of the straight-ahead or straight-backward running operation of the vehicle, i.e., irrespective of the rotating direction of the drive shaft 5. Since only a minute torque is applied to the two-way clutch mechanism to drive the hydraulic pump 26, the torque transmission capacity of the two-way clutch 32 can be decreased, whereby the miniaturization and cost reduction of the device can be attempted at being attained.