PURPOSE: To suppress low a measured value error by storing in advance an error ratio between detected data and operation processing result data, in a computer system, and correcting an error of the detected data by said ratio.

CONSTITUTION: Analog data is inputted 41 periodically to a data input device of a computer, processed 42 by an input processing program 42, and preserved in a raw data file in the computer. Subsequently, a count value is converted to an engineering value by an engineering conversion processing program, and it is preserved in an engineering value file 45. In this case, the computer is made to have a correcting information table 51 for storing an error ratio, and also a multiplying and dividing operation processing module 52 is provided so that the detected data is corrected by engineering unit converting processing 44 and integrating processing 46 based on data of said table 51. In this regard, data of said error ratio is obtained, for instance, by sampling and measuring a flow rate of water of each point for a prescribed period, and from this measured value and a measured value after the analog data has been processed.