  The concept of a self-consistent field in the theory of superconductivity
based on the diagram method of the time-dependent perturbation theory is
presented. It is shown that the well-known Bardeen-Cooper-Schrieffer equation
for the order parameter of superconductivity is already realized in a zero
approximation.The form of interaction Hamiltonian uniquely determines a chain
of interconnected Green's functions which are easily calculated in this
approximation. On the basis of the presented method a proximity effect in a
normal metal-superconductor structure is studied. It was obtained the energy
gap values induced in a normal metal. In contrast to the traditional McMillan
and de Gennes theories with self-consistent Green's functions the
self-consistency over the order parameter gives a significantly smaller gap
value induced in a normal metal. The frequency dependence of the homogeneous
spectral density is obtained which qualitatively agrees with experiment.
