To provide a water hammer preventing device 1 for damping and reducing water hammer or pulsation generated in a flow path, having improved damping performance while requiring a reduced installation space.

The water hammer preventing device 1 comprises a case 4 having an input port 6 communicated with an upstream side flow path 2A of the flow path 2 and an output port 7 communicated with a downstream side flow path 2B, a metal bellows 9 flexibly arranged in the case 4 and filled with gas 11, and a fluid passage 12 set in the case 4 with the input port 6 and the output port 7 communicated therewith and set to pass through the outer or inner periphery side of the metal bellows 9. It is installed directly on the midway of a main flow path for fluid as a feature.