Abstract: A filtration device for filtering and cleaning water to be supplied to a water tank comprises a main body having an upper end opening selectively closed by a lid mounted thereon in an installed state, a filtering unit detachably disposed in the main body, an air exhaust element communicating with an inside and outside of the main body, a water inlet connected to the main body, a feed water pipe connected to the water inlet for supplying water into the main body, a drainage pump for draining clean water cleaned up by the filtering unit, a water outlet operatively connected to the filtering unit to supply the cleaned-up water to the water tank, and a flow rate control unit, which is detachably incorporated into the main body and is connected to an inner portion of the water inlet, for controlling a flow rate of raw water, which is introduced from the water inlet to the main body, in a manner that a water level of water contained in the main body alternately has a high-tide water level, at which the filtering un