PURPOSE: To smooth filtration and dehydration operation, and facilitate mass production and machining, by forming the inner tube of screw press with wear resistant material corresponding to the changes in pressure resistance in gradual steps.

CONSTITUTION: The slurry supplied under pressure from a slurry inlet 2 is injected between a screw rod 6 and a low pressure inner tube 12, and is sent downward by the rotating screw rod 6, and is compressed by a screw part 7 of which bottom diameter becomes smaller as going lower, to be finally transferred to a high pressure inner tube 13 in the high pressure region. In this procss, the slurry is continuously filtered and dehydrated trough filtrate guide hole 9 in the inner tube 8, filter medium 10 and filtering cloth 11, and is collected in the outer tube 1, to be discharged from a filtrate outlet 17. The high pressure inner tube part 13 of the inner tube 8 is resistant to wear, and excels in mechanical strength, being fabricated with nontacky resin, so that trouble such as deformation and breakage due to abrasion never occurs. The dehydrated and compressed sludge becomes cake of 60W70% moisture content, and is discharged when a discharge valve 18 opens.