PURPOSE: To prevent a water level from substantially moving up and down about a critical level by providing a number-of-pumps control section, a primary control section for outputting a rotational speed commanding signal and a secondary control section for controlling the rotational speed so that the rotational speed commanding signal causes the whole drain flow to coincide with a set drain flow value.

CONSTITUTION: The controller is provided with a number-of-pumps control section 22 for outputting a pump driving signal to run the number of drain pumps 4 corresponding to a set drain flow value, a primary control section 23 for outputting a rotational speed commanding signal of a drain pump 4 on the basis of deviation of the whole drain flow from the set drain flow value and a secondary control section 17 for controlling the rotational speed of the drain pump 4 to a rotational speed commanding value. A controller 8 for a water level in a pump well is constituted such that the rotational speed commanding signal provides a signal causing the whole drain flow to coincide with the set drain flow value by making the rotational speed commanding value, represented by the rotational speed commanding signal, of the rotational speed of the drain pipe 4 with the operation of the secondary control section 17.