PURPOSE: To have a high efficiency rotary pump in a simple construction, which provides a wide utility, by allowing two rotary plate, fitted inside a cylinder air-tightly, to rotate alternately and differentially, and by attaining compression on the approaching sides of these two rotary plates and suction on their separating sides.

CONSTITUTION: Under the condition that a rotary plate 4 is situated ahead seen in the rotating direction (1) and that another rotary plate 5 is situated behind the plate 4, the former 4 is rotated at a specific high speed while the latter 5, at a low speed, and when the former 4 reaches the rear side of the latter 5, the rotation of 4 shall be made at high speed. Negative pressure is attained on the separating sides of the two rotary plates 4, 5 to cause suction of the fluid from a fluid suction hole 24, while a positive pressure is applied to their approaching sides, which leads to discharge the pressure fluid out of a discharge hole 26.