To solve the problem that the structure of conventional water hammer preventing devices is complicated since it is composed of bellows made of rubber members, thereby affecting the increase in the size and cost of the device.

When an increase in the pressure of a primary water flowing passage 22 occurs suddenly by quick closing a tap 2 and the pressure becomes a prescribed value or over, a piston 21 of the water hammer preventing device 1 is brought into opening motion against the energizing force of a spring means 24, and feeds the water from the primary water flowing passage 22 to the secondary water flowing passage 23. In such a way, the sudden excessive increase in the pressure in the primary water flowing passage 22 is prevented, thus avoiding the occurrence of the water hammer.