A filtration unit that includes a casing that has a closed bottom and is immersed in a water storage tank (or water storage pond) into which raw water is introduced, the casing being composed of a introduction port which introduces the raw water in the water storage tank into the casing, an active carbon supply device which adds active carbon to the introduced raw water, an agitator which agitates the raw water inside the casing to which active carbon has been added, a filtration module which filters the raw water, a back flow preventing device which prevents the raw water inside the casing from flowing back into the introduction port and being discharged into the water storage tank, and a discharging device which discharges the raw water from the casing.