To provide a line washing device for causing no trouble in an existing cooling medium pipe being used so far even in the case of a new cooing medium passing therethrough.

The cooling medium in a clean cooling medium cylinder 15 mounted on a cooling medium collecting device A is fed from a cooling medium feed line 22 into a line 21 to be washed. The cooling medium passing through the line 21 is entered from a collection pipeline 2 into a bypass line 25 and collected by a collection cylinder 30. A compressor 7, when operated, depressurizes a container 3 and also depressurizes the collection cylinder 30 connected thereto via the bypass line 25. The high pressure cooling medium discharged from the compressor 7 is fed through a bypass line 33 to the clean cooling medium cylinder 15 to increase pressure in the clean cooling medium cylinder 15. The cooling medium from the clean cooling medium cylinder 15 flows in the line 21 for washing operation and continuously collected by the collection cylinder.