PURPOSE: To secure a high reliability of self-suction ability simply by connecting a by-pass wheel formed axially to a casing through to a water reservoir and a suction port in a screw pump for which a pair of screw rotors are rotated in the casing.

CONSTITUTION: A suction side pipe projecting upwardly is coupled to a suction pipe 3 and a discharge side pipe projecting upwardly is also coupled to a discharge path 12, therefore water for self-suction is residual near a level W in a water reservoir 10 outside a discharge port 14. By providing an automatic water feeder constituted of the water reservoir 10 and a by-pass wheel 28 connecting the water reservoir 10 and a suction port 6 through each other which is provided axially to a casing 1, a high reliability of self-suction ability is simply secured, and a simple constitution of self-suction ability can also be secured for a screw pump with a large clearance between screw rotors 16 or the screw rotor 16 and the casing 1.