PURPOSE: To miniaturize and lighten the joint by changing the spool valve orifice opening area of a movale magnetic body by the movable magnetic body for generating attractive force according to the current of a solenoid, and a spring for generating load proportional to the displacement quantity.

CONSTITUTION: When a current is not applied to a solenoid coil 27, a movable magnetic body 28 does not generate magnetic attractive force, nor is generated the load of a spring 29, and an orifice 21 is fully opened so that oil flows to the orifice 21 and an intake passage 11 from a high pressure chamber 15 to place a torque characteristic in the free state. When the current is made larger and rotational difference is generated between a cam 2 and a rotor 3 in this state, a plunger supplies oil in a plunger chamber 5 into the intake passage 11, and the oil pressure of the plunger chamber 5 rises by the resistance of the orifice 21 to transmit torque between the cam 2 and the rotor 3. When the current is made further larger and the load of the spring 29 reaches the specified value, the movable magnetic body 28 comes in contact with a valve 9, so that the torque characteristic is placed into the lock state. The orifice opening can be thus continuously variable while miniaturizing and lightening.