PURPOSE: To secure a desired power transmission characteristic in an easy manner by constituting a variable orifice between a torsion bar spring, interposed between a forced circulation mechanism and a turning shaft, and circumjacent members, and selecting a torsional characteristic of this torsion bar spring properly.

CONSTITUTION: A forced circulation mechanism 4 and a joint member 5 are interposed between both first and second turning shafts 2 and 3. A gear mechanism 15 set up in this forced circulation mechanism 4 has a disklike holding part 17 and a stepped through hole 18 which consists of a large diametral part 18a and a small diametral part 18b. A torsion bar spring 42 is fitted in this small diametral part 18b of the through hole 18 so as to straddle a cylindrical part 5a of the joint member 5. When a speed difference is produced between front and rear wheels as well as a differential speed has occurred between both these first and second turning shafts 2 and 3, the torsion bar spring 42 comes to be twisted to the cylindrical part 5a on the basis of its characteristic in consequence, so that a passage sectional area in this part is varied to some extent, and thereby a variable orifice is formed there. In this connection, even when a torsional characteristic of the torsion bar spring 42 is selected, torque transmission can be changed.