PURPOSE: To ensure large orifice length in a simple structure, so as to choose different characteristics by forming an orifice body out of a first orifice body provided with a spiral wall part on a plate, and of a second orifice body of the same structure, both of which are superposed together in a wall part side.

CONSTITUTION: A spiral wail part 15 of a first orifice body 11 is stood on a plate 14 fixed by an annular bracket 13, on the peripheral wall of a casing 2, while a spiral wall part 17 of a second orifice body 12 is stood on a plate 16 almost in the same manner as the first orifice body 11. The wall parts 15, 17 are opposed with each other, and the upper end of each wall part 15, 17 is superposed on each plate 14, 16 in such a way that the upper end of the wall is abutted on the plate, so as to form an orifice body 10, and a spiral orifice 18 partitioned by the wall parts 15, 17 is formed between each plate 14, 16. At the time of running where engine shaking is generated, when the running state is detected by a car speed sensor, the second orifice body 12 is rotated by a controller 21, and the cross sectional area of the orifice 18 is reduced, and a small orifice is thus formed.