When the operator requests the output of performance data, if all the pieces of performance data in the requested time range could not be acquired, the output control section of each of supervisory control devices M1 to Mn will determine the cause. When the cause that all the pieces of performance data could not be acquired is the execution of zero suppression at nodes N1 to Nn, the data items related to the pieces of performance data that could not be acquired are made zero and the resulting data is outputted.