a filtration unit that includes a casing that has a closed bottom and is immersed in a water storage tank 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 .