inlet ports 22 24 are symmetrically formed in end heads 4a 6a respectively however outlet ports formed in the first embodiment are not provided in the end heads instead a fluid discharge hole 32a formed in a main body of a housing 2a directly opens to the rotor chamber 8 of course the hole 32a is opened at the rotor chamber 8 at the position substantially at the longitudinally center portion of the rotor 16 to prevent the vanes 20 from local wear on the other hand according to the third embodiment illustrated in fig5 outlet ports 26 28 are symmetrically formed in end heads 4b 6b respectively however inlet ports are not formed therein and instead a fluid intake hole 30a formed is in a main body of a housing 2b directly in communication with the rotor chamber 8 to introduce fluid from the fluid source thereinto the intake hole 30a is also positioned at the longitudinal center portion of the rotor according to the second and third embodiments vanes 20 are subject to a uniform fluid pressure at least during fluid intake operation in the second embodiment or at fluid discharge operation in the third embodiment to prevent the vanes from local wear however it is apparent that in the first embodiment having inlet and outlet ports at each of the end heads exhibits superior performance to that obtained in the second and third embodiments a fourth embodiment according to this invention is shown in.