PROBLEM TO BE SOLVED: To clean the interior of a heat transfer pipe.SOLUTION: A discharge pipeline 27 located at the discharge side of a condensate water pump 11, which discharges condensate water generated in a heat transfer pipe 4 to which steam is supplied, is branched to provide a branch pipeline 29 which introduces water discharged from the condensate water pump 11 into the heat transfer pipe 4 through a header 3. Then, cleaning water is circulated between the heat transfer pipe 4 and the condensate water pump 11 to clean the interior of the heat transfer pipe 4.