To achieve high vacuum by using a horn-shaped steam jet pipe at a second steam jet part to be connected to a suction pipe.

Gas molecules in a vacuum line are sucked to be introduced from a suction pipe 15 to a steam introducing P1P8 17 at a second steam jet part B. A second boiler 30 and a third boiler 40 are disposed inside this steam introducing pipe 17, and steam is jetted, so introduced gas molecules are mixed with steam. In the second and the third boiler parts 30, 40, horn-shaped jet pipes 11, 13 are provided. By the horn-shaped jet pipes 11, 13, steam is uniformly diffused while it is jetted to be mixed with the gas molecules. A taper angle of the horn pipe 11 is set at about 90° to include an axial line on a taper surface outside the horn pipe 11, and that of the horn pipe 13 is similarly set at about 64°. After they are mixed at the second steam jet part B, mixed matter is sent to a first steam jet part A, where it is mixed with setam jetted from a steam jet pipe 22, so it is supplied to an auxiliary pump such as a rotary pump.