PURPOSE: To prevent performance of an ejector from worsening by preventing a scale, mainly composed of silica contained in steam as drive gas supplied to a nozzle of the ejector, from sticking and accumulating in the nozzle.

CONSTITUTION: A nitrogen purge system 15 is provided to be connected to a three-way flow control valve 16, provided in the upstream spot of a steam supply system 8 connected to a steam generator 2 and a nozzle 5 of an ejector 4, and to a nitrogen supply system 11, and when stopped the ejector 4, the three-way flow control valve 16 is switched to flow nitrogen gas from the nitrogen supply system 11 via the nitrogen purge system 15 to the steam supply system 8 and into the nozzle 5, so that residual steam in these steam supply system and nozzle is purged to the outside of the nozzle 5.