PURPOSE: To prevent hydraulic pump power loss, etc., by making it possible to set the nipping force of a rim to a constant value utilizing the cross-sectional area of a differential valve together with providing a reliable actuation of the hydraulic circuit.

CONSTITUTION: Oiling from a hose 17a causes the second ejection hole 39 of a differential valve 27 to be closed, and the first one 26 to be opened, and thereby, a cylinder 11 for arm actuation is actuated to cause the arm 4 to be shifted to the cylinder 8 side allowing a rim 50 to be put between the tip of the arm 4 and a supporting part 41. When this supporting force reaches a prescribed pressure, the differential valve 27 is shifted causing the first and second ejection holes 26, 39 to be opened together. And thereby, oil flows in the cylinder 8 to cause the first valve 28 to be shifted to close the first ejection hole 26 and to thrust out a ram 42. By thrusting out the ram 42, a tire is pushed down to come off from the rim 50. When the oil pressure is removed, and device recovers. By this composition, the power loss can be reduced, and the tire can be surely disassembled.