To provide a buffer body for a water hammer preventer with its air chamber being easily manufactured and a water hammer preventer easily assembled with the buffer body.

A buffer body 190 for a water hammer preventer comprises a bottomed, substantially cylindrical elastic body case 20 and a syntactic form 19 filled and hardened in the elastic body case 20. The elastic body case 20 is formed with a narrowed part 21G formed by narrowing the diameter in the perpendicular direction to a water hammer direction in its any position in the height in the water hammer direction, and the syntactic form 19 is filled and hardened in the elastic body case 20 up to its opening edge along the inner diameter. The syntactic form 19 is thus reliably and easily filled into the elastic body case 20 through the opening provided thereabove, and the narrowed part is also easily formed. The buffer body 190 is then arranged to be contained in a topped cylindrical space part communicating with a water passage along its inner periphery to easily form a water hammer preventer having an air chamber S.