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PROBLEM TO BE SOLVED : To provide a water level-regulating system in which outflow of fertilizer and agrochemical from a field is minimized and adequate irrigation according to various conditions can be carried out and accumulation of earth and sand, etc. into an underdrain discharge pipe (a perforated pipe) can be suppressed. SOLUTION : The water level-regulating system is constructed by embedding the perforated pipe 21 in the ground of the field (cultivation section 13), installing service water-supplying part (a water supply pipe 22) for supplying service water to the perforated pipe 21 from a water supply channel on the upstream side and installing a water level setter 23 for setting water level of the field on the downstream side and installing an underdrain groove 24 in the direction crossing to the perforated pipe 21.