20 c and the sliding keys 2 are provided the diaphragm 7 and the head flange 5 can move only in a radial direction with respect to the casing 4 furthermore as shown in fig1 sliding keys 2 are provided at two portions in the peripheral direction so that moving directions of the two sliding keys 2 intersect with each other at the center of the rotating shaft 3 whereby the only relative movement held around the same center is allowed in consequence even when the casing 4 expands owing to an internal pressure and gaps between the casing inner peripheral surface 21 and the diaphragm outer peripheral surface 22 and between the casing inner peripheral surface 21 and the head flange outer peripheral surface 23 enlarge the casing 4 the diaphragm 7 and the head flange 5 can constantly hold the same center fig4 is a sectional view which is parallel to the rotating shaft and which shows the enlarged partial section a of fig2 as shown in fig4 when the abutment portions of the diaphragm 7 and the head flange 5 with respect to the casing 4 are provided with the sliding key 2 these three components can be held around the same center at the same time moreover an axial length la of the sliding key groove 20 a in the casing inner peripheral surface 21 is set to be larger than a sum of an axial length l of the sliding key 2 and an axial length lb of the key groove 20 b in the diaphragm moreover the axial length lb of the key groove 20 b in the diaphragm and an axial length lc of the key groove 20 c in the head.