PURPOSE: To reduce the necessary power and to eliminate damage due to mist, by spraying cooling liquid to steam prior to supply to a steam compressor and forming an unbalanced steam containing mist, while furthermore adjusting the grain size of the mist and feeding the steam.

CONSTITUTION: In a low pressure steam Sl supply path 2, a spray device 4 is arranged at the upstream side in an enclosed container 3 disposed at the inlet portion of a compressor 1, and cooling liquid L is sprayed into the low pressure steam Sl to form an unbalanced steam containing mist M. A demister 5 which can adjust the grain size of the mist M is arranged at the downstream side so as to recover the mist having large grain size thus feeding low pressure steam Sl containing only small grain size mist M. Since the mist evaporates sequentially in the compression stroke of the compressor 1, temperature rise of steam is suppressed and high pressure steam Sh of proper temperature is produced. Consequently, power consumption is reduced and damage of cylinder due to large grain size mist can be eliminated.