PURPOSE: To obtain steam of high purity and increase the life of a two phase liquid transportation pipe, by providing gas/liquid separation means for supplying steam in the two phase liquid transportation pipe at all times when a plant is at rest.

CONSTITUTION: When a plant is at rest, a little amount of geothermal fluid is yielded from a production well W through a shut-off valve V-1. The geothermal fluid is introduced to a gas/fluid separator S-2 through pipes 1, 5, a valve V-4 and a pipe 6 to be separated to steam and hot water. The steam separated is discharged to the outside of a system through a pipe 7, a valve V-5, pipes 8, 1-3, 1-4, a valve 6 and a pipe 9. With the steam flowing at all times when the plant is at rest, the protection film on the inside all of pipes can be prevented from being separated and the corrosion of the pipes is restricted.