  We investigate the duality in the Kondo model. Starting from the s-d model
with the coupling constant $J$, the strong coupling model with the constant of
1/J is identified. The model shows the unitary limit of the conductance, $G=2
e^{2}/h$ at zero temperature. The perturbation theory of the model gives
qualitative agreement with the results of numerical renormalization group and
Bethe ansatz near $T=T_{K}$.
