A screw conveyor for conveying an object to be conveyed, supported by a casing 1 by rotation of a screw 2, in which wear of a screw blade and decrease in conveyance efficiency are prevented without the need for extra power. Load on an electric motor 3 or the like for rotating the screw 2 or a value corresponding to the load is measured, and if the value is equal to or larger than a preset upper limit value, a screw shaft 2a is lifted up to increase a gap between an edge of the screw blade 2b and a bottom portion of the casing 1.