PURPOSE: To provide the inflow arithmetic unit which can calculate an inflow in conformity with actual water level variation.

CONSTITUTION: The inflow is calculated by a water level variation arithmetic program 31 when the frequency of water level variation is high, but when the frequency of the water level variation is low, the water level variation is checked at constant intervals of time and when no water level variation is found successively twice, it is considered that there is water level variation at the 2nd time, thereby calculating the inflow by a constant-time arithmetic program 30. Those two programs are controlled by an inflow arithmetic control program 32.