In a data acquisition system of ADC system, a log amplifier is provided at the pre-stage of an A/D converter, a signal amplified by the log amplifier having a nonlinear input-output characteristic is A/D-converted, and an adding operation of data is performed while reconverting a voltage value data which is converted to a nonlinear characteristic to data with a linear scale according to a table memory for reverse-log conversion. A known voltage value is inputted into the log amplifier to perform measurement, and calibration of the table memory is performed by storing the voltage value and the voltage value data after A/D-converted.