  An effective-field method for caculation of thermodynamic properties of
three-dimensional lattice spin models is developed. It is applied to the ANNNI
model on the simple cubic lattice. The phase diagram of the model, consisting
of a large number of commensurate phases and of an incommensurate phase, is
calculated, confirming the results of previous approaches. The phase transition
lines for a number of commensurate structures are localized, and a strong
evidence for absence of the direct phase transition between commensurate phases
and the disordered phase is found.
