PURPOSE: To easily and surely collect only the date that are suited to the simulation.

CONSTITUTION: When the operation data on a sewage treatment plant are collected as the simulation operation data, the data collection starting conditions are defined as far as the rainfall data serving as a main item exceeds the set value (x) among those data (S2) fetched at a data sampling time (S1) and also when a data collection carry-on flag is turned off (S4). Then the save (S6) acquired from the present data and the carry-on flag ere turned on (7) together with the data save (S5) covering past (m) hours. Thereafter the collection of data is carried on for a period longer than the carry-on hours (decrease) (n) despite decrease of the rainfall (S8, S9, S10). Thus the data correction starting conditions are defined as a piece of data on the main item so that the condition setting job is simplified. Then the data collecting conditions (x), the pest data collection hours (m) and the continuous data collection hours (n) can be freely set. Therefore the data can be collected in accordance with the plant scale and the conditions of the peripheral environment of the plant.