PURPOSE: To suppress the fluctuation in the display of a meter to a small extent and to permit accurate display, by charging an input signal into a capacitor, holding the same, amplifying the signal and driving the meter, and providing a switch which discharges the charge in the capacitor and a detection circuit for the level of an input signal.

CONSTITUTION: When a measurement signal S attains the certain specified voltage value or more set with a variable resistor 13, the output of a comparator 14 attains a logical value 1 and closes a switch 12. Then the charge in a capacitor 8 starts discharging according to a time constant. On the other hand, the signal S is amplified with an operational amplifier 6, and is charged through a diode 7 in the capacitor 8. When the signal S decreases from the specified value, the output of the comparator 14 attains a logical value 0 and opens the switch 13. The charge in the capacitor 8 is then discharged through a discharge resistor 9 and the switch 12 for the certain time alone but thereafter the charge is maintained at a certain value by the operation of a holding circuits consisting of the operational amplifiers 6, 10, etc. As a result, the fluctuation in the display on a meter 11 is decreased and the easy measurement of moisture content is made possible.