PURPOSE: To secure an accurate and quick recording for the characteristic change with age of the measured instruments by securing an automatic offset adjustment to subtract the absolute value output signal in case the characteristic change with age is measured for the measured instruments.

CONSTITUTION: With turning on of bias calibration switch 13 while measured signal eIN is supplied to comparator circuit 11 from the measured instrument, up-counter 15 is reset. And the counting is started from 0 with every input of clock signal CLK, and the count value is memorized and held. Accordingly, offset voltage ef of D/A converter 12 increases gradually. And the counting action is stopped when eIN< ef is secured, and the value of ef is held at converter 12. As a result, signal eIN and voltage ef of converter 12 are supplied to differential amplifier 19, and the absolute value output signal of the measured instruments is subtracted automatically to deliver voltage e0 showing the change with age.