  We analyze the Sun's shadow observed with the Tibet-III air shower array and
find that the shadow's center deviates northward (southward) from the optical
solar disc center in the "Away" ("Toward") IMF sector. By comparing with
numerical simulations based on the solar magnetic field model, we find that the
average IMF strength in the "Away" ("Toward") sector is $1.54 \pm 0.21_{\rm
stat} \pm 0.20_{\rm syst}$ ($1.62 \pm 0.15_{\rm stat} \pm 0.22_{\rm syst}$)
times larger than the model prediction. These demonstrate that the observed
Sun's shadow is a useful tool for the quantitative evaluation of the average
solar magnetic field.
