PURPOSE: To give a function weakening a water hammer by a simple structure by inserting a sleevelike member made of an elastic material into the body of a high pressure water jetting device and forming a through hole opening by a pressure of passing through water at the nozzle top.

CONSTITUTION: A sleevelike member 9 is inserted into the body 6' of a high pressure water jetting device. The sleevelike member 9 is made of an elastic material such as rubber and a valve body 9b provided with a prescribed diameter through hole 9a which opens by a passing through pressure of high pressure water and which closes by shut of the water is built in the member 9. The inside wall of a nozzle is equipped with an inside protecting layer 9c made of an elastic material and formed integrally with the valve body 9b. High pressure water flows into the high pressure water jetting device 8 when a rolling stock 7 passes. In that time, the inside protection layer 9c deforms elastically to protect the inside of the device 8 from stresses caused by a sudden pressure change. Also the valve body 9b deforms elastically in the periphery direction so that the through hole 9a opens and water being in the nozzle jets from the nozzle top.