A torque characteristic shifting mechanism is provided in an accommodation hole of a rotary valve. The torque characteristic shifting mechanism changes over the torque characteristics in a multi-step manner with an increase of the vehicle velocity by setting the distances so as to meet the relationships R3≈R2>R1 or R2>R3>R1 where R1, R2 and R3 are the distance from a center of rotation of a weight member accommodated rockably around the center of rotation to a center of the ball blocking the drain hole, the distance from the center of rotation to a center of gravity of the weight member and the distance from the center of rotation to a center of a spring, respectively.