There is disclosed a method of accurately calculating the magnetic field strength at an arbitrary position in an ESR (electron spin resonance) spectrum using Mn2+ marker. The spectrum contains an ESR signal arising from an unknown sample and an ESR signal arising from a reference sample containing Mn2+ marker. The resonating magnetic field strengths Hci and Hcj of two absorption lines of the six absorption lines arising from Mn2+ are calculated from the applied microwave frequency, the nuclear spin quantum numbers mIi and mIj producing the two absorption lines, a predetermined g value go intrinsic to Mn2+, and a predetermined isotropic hyperfine coupling constant A. The magnetic field strength Hcx at a position of interest in the ESR spectrum is determined from the distance between the absorption lines, the distance between the position and one of the two absorption lines, and the magnetic field strengths Hci, Hcj.
