PURPOSE: To take an accurate measurement of strain by connecting circuits which detect a voltage 90° out of phase with a source voltage and canceling voltage generating circuits with a canceling function for an error component to the output circuit of a bridge because the error component due to the stray capacity of the bridge circuit when a measurement of strain is taken by using the bridge circuit.

CONSTITUTION: Circuits 61 and 62 which detect a voltage 90° out of phase with the source voltage of an AC power source 2 and canceling voltage generating circuits 81 and 82 which have a function for canceling the error voltage due to the stray capacity of the circuit corresponding to the level of the voltage are connected to the output circuits 41 and 42 of the bridge. Further, canceling voltages E1 and E2 are fed back to the output circuits 41 and 42 through capacitors 91 and 92. Consequently, even if the stray capacity is present, any output voltage due to that is not generated, so an accurate measurement of strain is taken by any kind of bridge constitution circuit.