faces of the key gutter 10 therefore the key 11 is fixed so as to be prevented from moving in the axial direction lastly the key securing bolts 18 are turned to be tightened to completely firmly fix the key 11 to the lower outer casing 112 according to the centering mechanism 1 of this embodiment the key 11 is only moved in the key gutter 10 in the axial direction so that the lower inner casing 113 is moved along the oblique member 16 provided for the key 11 in the direction perpendicular to the axial direction i e in the vertical direction in a non stepwise manner therefore it is possible to easily and quickly position the upper inner casing 111 and the lower inner casing 113 in the vertical direction to improve the efficiency of the centering work and to reduce the work time unlike the conventional technology it is unnecessary to prepare a new key liner every time the centering work is performed therefore the cost and the work time required for the centering work can be substantially reduced the inclination angle of the oblique member 16 is set such that low pressure side end faces 111 a and 113 a of the upper inner casing 111 and the lower inner casing 113 are brought into contact with are pressed against high pressure side end faces 110 a and 112.