  The two-point Green function of the massive scalar $(3+1)$-quantum field
theory with $\lambda\phi^4$ interaction at finite temperature is evaluated up
to the 2nd order of perturbation theory. The averaging on the vacuum
fluctuations is separated from the averaging on the thermal fluctuations
explicitly. As a result, the temperature dependent part of the propagator is
expressed through the scattering amplitudes. The obtained expression is
generalized for higher orders of perturbation theory.
