Screw type pump

A screw pump having a pair of screw rotors (8) meshed with each other and rotatably supported in a housing (6) having an intake port provided at one end and a piston (31) movably provided at the other axial end of the housing, which piston (31) defines a discharge port (5) in cooperation with the housing. The movement of the piston in a direction substantially perpendicular to the axes of the screw rotors enables the distance between a discharge starting position (PE,PE') of the discharge port and the intake port to be varied, so that the internal compression ratio can be varied. This arrangement ensures that the size of the housing will not be increased and any disadvantage due to the difference in thermal expansion along the length of the housing can be overcome.