A screw conveyor for the transport of flowable substances and/or lumps of material comprises a rotatable first screw (1) provided with helical windings (2) arranged in a housing (3) with an inlet opening and an outlet opening. The housing (3) comprises an inlet chamber (4) and a pumping chamber (5) between said inlet and outlet openings and at least in the pumping chamber (5) a second screw (6) is provided with helical windings (7) and arranged for rotation in the opposite direction of the first screw (1), said screws (1, 6) in said pumping chamber (5) providing a positive displacement pumping function by mutual engagement between the two screws (1, 6). By having the parts of the two screws (1, 6) in mutual engagement extending a distance corresponding to at least one half winding out of the pumping chamber (5) and into the inlet chamber (4), overfilling of the pumping chamber (5) and accordingly, the subsequent increased friction in the pumping chamber (5) is avoided.