PURPOSE: To simplify the structure of a rotary pump which carries out less pulsative discharge, by disposing a rotor rotatably in a rectangular pump chamber such that the center of the rotor traces a true circular path, and by meshing external teeth in a rotor shaft always with internal teeth in the center section of the rotor.

CONSTITUTION: Suction ports 4a through 4d and discharge ports 5a through 5d are formed, respectively in the corner parts 3a through 3d of a square-like shape pump chamber 2 defined by inner walls 2a through 2d having a side length L, and check valves 6, 7 are disposed each suction and discharge ports, respectively. The rotor shaft 10 is rotatably journalled at the center of the pump chamber 2, and an equilateral triangular-like shape rotor 50 is rotatably disposed in the chamber 2. This rotor 50 is formed such that arcs a' through c' having a diameter of L are depicted about the apexes A' through C' of an equilateral triangular 11 having a side length L, and the area defined by these arcs a' through c' is used as the shape of the rotor 50. With this arrangement, the rotor 50 may be made always in contact with the inner walls 2a through 2d of the pump chamber 2 at four points, and external teeth in the rotor shaft 10 is meshed always with the internal teeth in the center part of the rotor 50.