To keep the output of the blower of a diffuser low while securing the maximum pondage and maximum water depth for the seepage water regulating tank.

A part of the effective water surface in a tank is partitioned, and the region in the tank is divided by a partition wall 12 for partitioning the effective water depth in the tank close to the middle into an upper normal storage region 13 and a prestorage region 14 other than the normal storage region, an overflow weir 15 is provided for the partition wall to communicate the normal storage region with the prestorage region, a first diffuser 16 is arranged close to the bottom of the normal storage region, a second diffuser 17 close to the bottom of the prestorage region and a third diffuser 18 close to the middle of the effective water depth in the prestorage region, and a discharge pump 24 for discharging the seepage water outside the tank and a circulating pump 25 for supplying the seepage water in the prestorage region to the normal storage region are furnished.