To provide a water hammering preventing device for a motor-driven pump capable of preventing occurrence of water hammering without using any flywheel owing to the slow stop function of an inverter, wherein the effect functions effectively even in the event of power failure.

The motor-driven pump 7 operated by AC waveform prepared by an inverter part 6 using an inverter device 1 including a converter part 3 and inverter part 6 is equipped with a DC auxiliary power supply circuit through which the DC auxiliary power accumulated in an accumulating power supply 8 via a charger part 9 upon branching at the AC source 2 is fed to the inverter part 6 via a power changeover switch 10, and when the pump is stopped as normal operation, an operation command switch 5 is put off so that the pump 7 is decelerated and stopped gradually by the slow stop function of the inverter, and in the event of power failure, the power changeover switch 10 is put on so that the DC auxiliary power is supplied, while the operation command switch 5 is put off so that the pump 7 is decelerated and stopped gradually by the slow stop function of the inverter.