PURPOSE: To design a device itself in a compact size, and also improve its reliability.

CONSTITUTION: A steam separator is provided with a valve box 14 in which the opening of a discharge pump side pipe passage 9a is connected to the lower part thereof and the opening of the vacuum pump side part pipe passage 9b is connected to the upper part thereof, and a valve body 15 which is slided in vertical direction in the valve box 14 and which has a small hole 16 for communicating the opening port of the intake pipe 9a connected to a part which is sliding contact with the valve box 14 and/or the lower part of the valve box 14, to the opening port of the intake pipe 9b connected to the upper part of the valve box 14. In the case where suction of water is started by a vacuum pump 3 for priming, the valve body 15 is lifted upward by flowing resistance when water passes through the small hole 16 of the valve body 15 so as to seal the opening port of the vacuum pump side pipe passage 9b connected to the upper part of the valve box 14.