To provide an energy saving type rotary pump utilizing buoyancy as rotation power.

This pump has a configuration for rotatably arranging a rotary floating body 12, having as a buoy, a cylinder 19 arranged in a horizontal direction and at least one piston 21, 22 slidably built into the cylinder, and filling the inside of the cylinder with gas by sliding the piston in a reverse direction while introducing liquid to the inside of the cylinder by sliding the piston in one direction. For sliding the piston, rods 25, 26 projected to the outside of the cylinder are provided, and guide rails 31, 32 arranged along the circumferential direction of the rotation of the cylinder and the rods are connected, thereby, the piston is guided with rotation of the rotary floating body.