A screw conveyor includes a plurality of shafts rotatable in a common imaginary plane, and a plurality of blades arranged on each of the shafts in an axial direction of the shafts such that the blades define a part of an imaginary spiral around the shafts, each of the blades being semicircular in shape when viewed in an axial direction of the shafts, a distance S being longer than a length D, but shorter than 2D, wherein “S” indicates a distance between outer surfaces of two shafts located adjacent to each other, and “D” indicates a length by which the blades extend from outer surfaces of the shafts.