We analyze the Suns shadow observed with the Tibet-III air shower array and find that the shadows 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 Suns shadow is a useful tool for the quantitative evaluation of the average solar magnetic field.
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