The present invention provides a magnetic flux measuring device and magnetic flux measuring method for stationary induction electrical apparatus which can accurately measure the magnetic flux without being influenced by the DC offset component which superimposes on the measurement system, and can constantly and continuously measure the magnetic flux without setting a limit in the magnetic flux measurement period. The magnetic flux measuring method which measures a magnetic flux of a stationary induction electrical apparatus 2 using a voltage waveform of the stationary induction electrical apparatus 2, wherein the magnetic flux 151 of the stationary induction electrical apparatus 2 is determined by the steps of: inputting a voltage waveform 121 of the stationary induction electrical apparatus 2 into a negative-feedback amplification means 14 of which feedback circuit is constructed by a low pass filter; and integrating output of the negative-feedback amplification means 14 by integration means 15.