PURPOSE: To detect accurately the fluctuated amount of the bridge without using any high-stability circuit element, by controlling the current to be supplied to the bridge according to the time proportional to the output of the bridge and then counting the clock pulses within the proportional time.

CONSTITUTION: The current I1 is supplied 11 to the resistance bridge 1, and the current I2 having a certain ratio to the current I1 is supplied via the switching element 13 in the direction where the output of the bridge 1 is degated. Then the time signal proportional to the phase compensation output is generated 27 after the amplification and the phase compensation 20 given to the output voltage of the bridge 1. And thus the element 13 receives the switching control 31 based on the time signal mentioned above. At the same time, the clock pulses 28 are counted 33 via the gate 32 and during the duration of the time signal and then displayed 10. As a result, the fluctuated amount of resistance can be displayed for the bridge 1.