PURPOSE: To prevent the deformation of a blade while keeping smooth access to a groove in the blade by providing a rotor, which is formed of softer material than for the blade, in an area in contact with the blade at least and plural operating rooms partitioned with the blade.
CONSTITUTION: A piston 7 is made of metal, e.g. in most part and higher- modulus, softer material than for a blade 8 only in an area 21 where a groove 6 is formed. On the other hand, the blade 8 is formed in a sectional rectangle, e.g. and fitted into the groove 6 in the piston 7. The blade 8 has the outer periphery put in contact with the inner periphery of a cylinder 5 with air tightness to form plural operating rooms 9, which vary gradually smaller in capacity from the suction side to the discharge side of the cylinder 5, in a winding space. The blade 8 is put in access to the groove 8 corresponding to the rotations of the cylinder 5 and the piston 7 in opposition while being subjected to differential pressure between the adjacent operating rooms 9, 9, and made to move in and out in a radial direction to the piston 7 while being put in contact with the area 21 where the groove 6 is formed.