PURPOSE: To prevent metal education at liquid contact parts as well as prevent abnormal vibration of the pump by making the liquid contact parts of a casing etc. from plastics, providing the convolution chamber with two discharge ports, and by forming this convolution chamber in point-symmetrical configuration.

CONSTITUTION: The liquid contact parts of discharge casing 8, suction casing, etc. of a pump are made of anti-corrosive plastics. Therefore metal education at these liquid contact parts can be prevented, and discharge ports 11, 11' are provided at two points in the discharge casing 8 point-symmetrically with the convolution chamber as its center, wherein the convolution chamber between its flap parts 14, 14' and discharge ports 11, 11' is formed also point-symmetrically, so that the left and right stresses applied to the pump shaft 4 are well balanced to accomplish prevention of eccentricity of the pump shaft 4 likely to be produced by umbalance of the left and right stresses. Accordingly use of any intermediate underwater bearing can be eliminated even when a long pump shaft is required.