PURPOSE: To provide a convenient and inexpensive pump having a constant discharge volume and well-balanced forces caused by the generation of hydraulic pressure and acting upon several parts.

CONSTITUTION: The shape of the inner peripheral surface of a casing is set to be symmetric with the major axis M1-M2 and the minor axis Q1-Q2, and has a guide surface 2b formed of a continuous curve consisting of non-circular and arc parts in which a variation in radius of a quarter circle is proportional to a variation in angle, so that a pump chamber 10 is formed. A valve cylinder 6 in which discharge recesses 8A, 8B and suction recesses 9A, 9B are alternately arranged at positions forming an angle of 45 deg. from the major and minor axes, is fixed to a casing 2, coaxially therewith, and A ring-like rotor 3 having three ball plungers each of which is composed of a circular hole 4 and a ball 5a and which are laid at positions obtained by angularly equidividing the ring- like rotor 3 is driven in the pump chamber 10. Since the action of the three ball plungers is divided by an angle of 30 deg., and accordingly, it is possible to uniformly introduce and discharge fluid.