PURPOSE: To intend to prevent bite in of foreign substance and to improve reliability by setting diameters of screws adjacent to the intake port of a screw conveyer smaller than screw diameters after the intake port, in a screw pump for carrying sludge such as the sea bottom.

CONSTITUTION: A screw pump 1 is composed of a screw conveyer 2 having an intake port 6 on its bottom end and of a pump 3 having a delivery port 15. The diameter of a screw 8a adjacent to the intake port 6 is set smaller than diameter of a screw 8b after the intake port 6. In this case, diameter of the screw 8a is set in a range of about 40-70% of diameter of the screw 8b. Moreover, the blade 3a of the pump 3 is enlarged in the diameter it forward the delivery side, and a casing 3c covering the blade 3a is formed in an annular shape being expanded at the lowermost stream. And the delivery port 15 is formed in tangential direction of this annular casing 3c. According to this constitution, bite in of foreign substance is prevented and improvement in reliability can be intended.