  We analyze the unanalytical structure of metal-insulator transition (MIT) in
infinite dimensions. By introducing a simple transformation into the dynamical
mean-field equation of Hubbard model, a multiple-valued structure in Green's
function and other thermodynamical quantities with respect to the interaction
strength $U$ are found at low temperatures. A unified description of stable,
metastable and unstable phases is obtained in the regime
$U_{c1}(T)<U<U_{c2}(T)$, and the Maxwell construction is performed to evaluate
the MIT line $U^{\ast}(T)$. We show how the first-order MIT at $U^{\ast}(T)$
for $T>0$ evolves into second-order one at $U_{c2}(0)$ for $T=0 $. The phase
diagram near MIT is presented.
