PURPOSE: To prevent the integration of measuring errors and to improve the reliability of a flow rate and the like, by performing the A/D conversion of the instantaneous value of a read analog signal, correcting the error, and thereafter integrating the value.

CONSTITUTION: A flow rate signal from a flowmeter 1 is processed by a program in a controller 14. Reading of the instantaneous value of the flow rate signal, A/D conversion of the read analog signal, and correction of measurement errors are performed. The digital values, whose measurement errors are corrected, are integrated, and the result is displayed on a digital display 16. By the integrated value, whose measurement error integration is prevented, the reliability of the integrated value of the flow rate and the like is improved.