PURPOSE: To enhance efficiency by providing the inside of a cylinder with an elliptical rotating body suited to the cylinder, and on both sides of the circumference, by symmetrically installing semicircular rotating bodies via rollers respectively, and also by symmetrically fitting semicircular cylinders suited thereto on both sides on the circumference of the cylinder.

CONSTITUTION: When a liquid used in a device that is all symmetrical on both sides is a negative-pressure liquid, if the vertex of an elliptical rotating body 1 passes a liquid inlet chamber 9, a rotary valve 10 is opened, and by the attractive force of negative pressure a roller 3 is brought into close contact both with the surface of a semicircular rotating body 4 and with that of an elliptical rotating body 1 to function as a pressure separation zone. And the roller 3 obtains a moment of inertia from the liquid pressure applied to the sectional integration connecting the contact points of both the rotating bodies 1 and 4 being closely in contact with each other, and since the surface angle on the elliptical rotating body 1 side is not perpendicular to the center line of the turning shaft 6, force is dislocated, and the front half pushes the elliptical rotating body 1 in the turning direction. Since the liquid pressure is separated by the vertex of the rotating body 4 and by the wall of a cylinder 2 and rollers 3, the rotating body 1 obtains a moment of inertia from the liquid pressure and operates until it reaches a liquid-discharge chamber 8.