A strain measuring device using a Wheatstone bridge circuit. An excitation voltage changing switch switches from one excitation voltage to a different excitation voltage in accordance with a controlling circuit providing sequence operation of them and of other circuit elements to remove spurious components affecting the measurement of strain. Such spurious components include potential errors due to thermal electromotive force produced inside or outside said Wheatstone bridge circuit and sensitivity error due to drift voltage occurring in the driver circuit of the bridge current source. A comparator may be used to effect judgment of the amount of the measured value of the voltmeter, so that the controlling circuit causes excitation voltage changing switch to select only two excitation voltages out of at least three for switching no matter how many excitation voltages are provided for in the bridge current supply source.
