In the case of μ* = 0, the following Tc formula is derived analytically from the Eliashberg equation Tc=αωlogexp-b((1+cλ)/λ), where, α = 2γ/π, b = c =1 and In γ = C = 0.5772 which is the Euler constant.The formula obtained by the authors holds only when Tc is less than 0.36/ α (K), where a is a constant larger than 1 and differs from one material to another. We conjecture, it is very likely that, when Tc is larger than 0.36/a, the functional structure of the Tc formula differs from that of McMillan's or at least the parameters a, b, and c are no longer constants independent of materials.