In this paper, we propose the estimating method of the magnetic field strength waveforms under the distorted flux density conditions. The estimated magnetic field strength, which is called as EFS, calculated by the magnetic field strength waveform that can be obtained when the specimen is excited with a fundamental sinusoidal frequency component of the distorted flux density waveform. We compare the estimated field strength waveform with the field strength waveform measured under the distorted flux density condition directly, which is called as MFS. Finally, we evaluate the efficiency of EFS.