PURPOSE: To prevent to generate troubles in the operation of the heat pipe by a method wherein a bypassing side path is provided between a steam communicating pipe and a condensed liquid communicating pipe to flow drain generated in the steam communicating pipe to the side of condensed liquid communicating pipe.

CONSTITUTION: A drain communicating pipe 9 is provided between the steam communicating pipe 7 and the condensed liquid communicating pipe 8 to flow the drain, generated in the steam communicating pipe 7, into the condensed liquid communicating pipe 8. A pressure reducing valve 10, a U-shape pipe 11 filled with operating liquid and a blow valve 12 are connected in series on the way of the drain communicating pipe 9, an overflow pipe 13 is connected to the other end of the U-shape pipe 11 and the drain in the steam, in which the steam and the liquid are mixed, is discharged by the pressure reducing valve 10 while the drain is bypassed by the overflow pipe 13 into the condensed liquid communicating pipe 8. The blow valve 12 is fully opened upon beginning the operation of a boiler or the like together with the pressure reducing valve 10 to discharge the rust or the like of the path of pipelines to the air. In another way, a drain trap 14 and the pressure reducing valve 10 are connected in series to the drain communicating pipe 9 and a valve 15 for maintenance is connected in parallel to these trap and valve.