Abstract: The water purification device includes at least a purification unit and a pipe unit. The water purification device is intended for purification of water being particularly in a storage, such as a container, a reservoir, a vessel or the like, by immersing the same at least partly into the water existing in the storage and by leading water away from the storage preferably from above into a collecting container placed essentially underneath the purification unit. A pipe connected to the purification unit is arranged to lead water by means of syphon principle, by influence of syphon pressure caused by the difference between the height of the purification unit and the collecting container. The filtering means include at least one thin filtering film, preferably made of plastic-based material, such as PTM (Particle Trade Membrane), TeM (track-etch Membrane) or the like, the thickness of which is preferably 7-25 micrometers and the size of pores is 0.05-10 micrometers.