  An analytic method for deriving the free energy of a three-dimensional
Ising-like system near the critical point in a homogeneous external field is
developed in the $\rho^6$ model approximation. The mathematical description
proposed for temperatures $T>T_c$ ($T_c$ is the phase-transition temperature in
the absence of an external field) is valid for fields near $\tilde h_c$, where
the scaling variable is of the order of unity and power series in this variable
are not effective. At the limiting field $\tilde h_c$, the temperature and
field effects on the system in the vicinity of the critical point are
equivalent. The total free energy is obtained as a function of temperature,
field and microscopic parameters of the system without using series expansions
in the scaling variable. In addition to leading terms, the expression for the
free energy includes the terms determining the temperature and field confluent
corrections.
