PURPOSE: To discharge water in a device rapidly so as to restart a vacuum pump for priming easily by blocking the opening port of the priming vacuum pump side by a valve body when priming is sucked in a valve box, and also accordingly opening the inside of the valve box toward the atmosphere by a valve mechanism.

CONSTITUTION: A steam separator 11 is provided on the way of intake pipes 9a, 9b for connecting the suction side of a discharging pump 2 to the suction side of a priming vacuum pump 3. In this case, the valve box 14 of the steam separator 11 is connected respectively to the opening port 16 of the intake pipe 9a communicated with the discharging pump 2 side, the opening port 19 of the intake pipe 9b communicated with priming vacuum pump 3 side and the opening port 18 of an exhaust pipe 17 provided with a valve 12. A valve body 15 for blocking the opening port 19 when priming is sucked is provided inside the valve box 14. And also, a valve mechanism 22 is provided to open the inside of the valve box 14 to the atmosphere while being interlocked to movement of the valve body 15. The valve mechanism 22 comprises a passage 23 drilled on the upper part of the valve box 14, a blocking valve 24 for blocking the passage 23, a piston 25 for opening/closing the valve 24, and the like.