  Partial summing of infinite range of diagrams for the two-phonon mass
operator of polaron described by Fr\"{o}hlich Hamiltonian is performed using
the Feynman-Pines diagram technique. Renormalized spectral parameters of ground
and first excited (phonon repeat) polaron state are accurately calculated for a
weak electron-phonon coupling at $T=0$ K. It is shown that the stronger
electron-phonon interaction shifts the energy of both states into low-energy
region of the spectra. The ground state stays stationary and the excited one
decays at a bigger coupling constant.
