To provide a pump-priming device capable of priming without using troublesome works, and causing little inhibition of water circulation in the case of circulating storage tank water by operating a circulation pump, and to provide an external filtering apparatus furnished with the pump-priming device.

The pump-priming device for performing the priming work to a circulation pump 54 placed at a level lower than the water level of a storage tank is provided with a priming pump 6 placed in a bypass channel 10 formed in the middle of a flow channel 4 connected to a water-supply port of the circulation pump 54, and a check valve 8 placed in a flow channel 12 adjacent to the bypass channel 10. The check valve 8 is closed to close the flow channel 12 adjacent to the bypass channel 10 when the priming pump 6 is operated to flow the storage tank water through the bypass channel 10 to the circulation pump 54, and the check valve 8 is opened to flow the storage tank water through the flow channel 12 adjacent to the bypass channel 10 to the circulation pump 54 when the circulation pump 54 is operated to suck up the storage tank water.