  A new formulation of perturbation theory for a description of the Dirac and
scalar fields (the Yukawa model) is suggested. As the main approximation the
self-consistent field model is chosen, which allows in a certain degree to
account for the effects caused by the interaction of fields. Such choice of the
main approximation leads to a normally ordered form of the interaction
Hamiltonian. Generation of the fermion mass due to the interaction with
exchange of the scalar boson is investigated. It is demonstrated that, for zero
bare mass, the fermion can acquire mass only if the coupling constant exceeds
the critical value determined by the boson mass. In this connection, the
problem of the neutrino mass is discussed.
