  Possibility for coexistence of a phonon and one-particle spectra in ${}^4He$
at all temperatures including temperatures below the $\lambda$ - point is
shown. An approach developed to consider the problem of the weakly imperfect
Bose gas results in the existence of a gap in the one-particle (one-atom)
excitation spectrum below the transition point. Thus, the microscopic model of
He II turns out to differ both from the model initially suggested by Landau and
from the Bogolyubov model accepted at present. Analysis of the experimental
data obtained by measuring enthalpy and the part of helium atoms condensed in
the state with the momentum $p=0$ enabled to estimate the size of the gap. It
turned out to be approximately equal to 0.31 K at zero temperature.
