PURPOSE: To reduce the installation space of a water purification device and purify a contaminated water economically by a method wherein the water purified in each of purification cartridges is collected for sending as a clean water.

CONSTITUTION: A contaminated water is supplied from a water feed pipe 1 into each of a plurality of purification cartridgers 4, through which this water is filtered. The water thus purified is collected for discharge from a water supply pipe 9. To clean the inside of the purification cartridge 4, a clean water is supplied thereinto from the water supply pipe 9 and this clean water is made to flow backward inside the purification cartridge 4 and discharged from the water feed pipe 1. In this way, the substances adhered to a filter 15 are separated therefrom for discharge to clean the inside of each purification cartridge 4.