PURPOSE: To effectively prevent occurrence of the gear noise from a differential gear by composing a delivery port so that it will not be completely closed by a closing member in virture of the resiliency of an elastic member when the relative rotation difference between a rotor and a cam ring is small.

CONSTITUTION: When a rotor 10 makes a differential motion in the arrow direction against a cam ring 12 due to the generation of a rotation difference between a front wheel and a rear wheel, an operation oil 13 in a chamber 22a inside the right side of an actuating chamber 22 is pushed by a vane 11 and the pressure in the chamber 22a on the right side of the vane 11 becomes bigger than the pressure in a chamber 22b on the left side. Together with this phenomenon, the operation oil 13 in a oil reservoir 21 run into the left chamber 22b through an inlet port 14 while a closing member 24 in a delivery port 15 is pushed toward a closing valve resisting to the energization force of a spring 25. Due to this reason, the delivery port 15 will be in a throttled condition when the rotation difference between the front and the rear wheels is small and the pressure in the delivery port 15 is small.