To provide a sand filtration system based on the slow sand filtration method, preferably cleaning even highly turbid water.

The sand filtration system is of a two-stage type with a first filtration tank 11 and a second filtration tank 12 serially connected. The filtration rate of the first filtration tank 11 is set faster than that of the second filtration tank 12, for example, is set 4 times of the filtration rate of the second filtration tank 12. Primarily filtered water from the first filtration tank 11 is fed to the second filtration tank 12, wherein the filtration rate of the first filtration tank 12 is slower than that in the first tank 11, and a part of the water fed is normally returned to a precipitation tank 22 through an overflow return pipe 21 from an overflow port 20. The occurrence of scum in the second filtration tank 12 is suppressed, and the primarily filtered water from the first filtration tank 11 can be preferably filtered in the second filtration tank 12 by the slow sand filtration method.