  We study how to recover the unitarity of Lee model with the help of
bi-orthogonal basis approach, when the physical coupling constant in
renormalization exceeds its critical value, so that the Lee's Hamiltonian is
non-Hermitian with respect to the conventional inner product. In a very natural
fashion, our systematic approach based on bi-orthogonal basis leads to an
elegant definition of inner product with a non-trivial metric, which can
overcome all the previous problems in Lee model, such as non-Hermiticity of the
Hamiltonian, the negative norm, the negative probability and the non-unitarity
of the scattering matrix.
