PURPOSE: To slow air discharge to reduce the rate of speed increase inside pipe lines so as to prevent abnormal pressure buildup in a pipe line water- hammer prevention device that uses air valves by making air discharge resistance value between an air discharge means and a pipeline main greater than an intake resistance value between an intake means and the pipeline main.

CONSTITUTION: In a water-hammer prevention device comprising a feed water pipeline main 1, air intake valves 21, 22 and air release valves 31, 32 all of which are communicated with one another by a communicating pipe 4, and a check valve 41 interposed between the main 1 and each of the large-bore air suction valves 21, 22, the bore of each air release valve 31, 32 is made smaller than that of each air suction valve 21, 22 so that an air discharge resistance value is enhanced. That is, air is released slowly by increasing resistance during air discharging, and a pressure buildup is lowered the moment a flow of water is stopped, so as to eliminate the possibility of abnormal pressure buildup.