A thread-groove pump mechanism portion PB employs a turn-back structure including a rotor (18) formed of a multiple cylinder having an inner cylindrical rotor (18-1) and an outer cylindrical rotor (18-2) and a stator (24) formed of a multiple cylinder having an inner cylindrical stator (24-1) and on outer cylindrical stator (24-2). Gaps g1 and g3 defined by the outer walls of the rotor and the stator walls, and a gap g2 defined by the inner cylinder wall of the rotor and the stator wall during the rest of the pump are formed such that they increase with the distance from the rotor shaft center and g1 > g2 and g1 > g3 are satisfied. Thus, even if displacement occurs by the centrifugal force and thermal expansion during the operation of pump, predetermined gaps can be provided therebetween. <IMAGE>
