  The paper presents new method for calculating the low-temperature asymptotics
of free energy of the 3D Ising model in external magnetic field $(H\neq 0)$.
The results obtained are valid in the wide range of temperature and magnetic
field values fulfilling the condition: $[1-\tanh(h/2)]\sim\epsilon,$ for
$\epsilon\ll 1$, where $h=\beta H$, $\beta$ - the inverse temperature and $H$ -
external magnetic field. For this purpose the method of transfer-matrix, and
generalized Jordan-Wigner transformations, in the form introduced by the author
in $\cite{mkoch95}$, are applied.
