PURPOSE: To obtain a gate effluent rate with high accuracy without the needs to increase the capacity of a main memory and also to delay the processing speed by a method in which all numeric tables are housed in a large-capacity memory and necessary portions are sent to the main memory.

CONSTITUTION: A large-capacity memory 14 to house all numeric tables 141 is provided. A given range of numeric data including the present level of stored water are read out of the table 141 of the memory 14 and sent to numeric tables 132 and 133 in the main table 13 by a numeric data transmitter 12. A gate effluent rate arithmetic means 12 is also provided to obtain gate effluent rates by using the tables 132 and 133 in the main memory 13. Since a part of the tables stored in the memory 14 is matched with the present water level to be processed and sent to the main memory 13, the gate effluent rate can be obtained with high accuracy without delaying of processing speed.