  The calculation of the charge present in central region of the double barrier
structure at non-equilibrium conditions is discussed. We propose here a simple
method to calculate non equilibrium Green's functions which allows consistent
calculations of retarded and distribution functions. To illustrate the approach
we calculate the charge on the quantum dot coupled {\it via} tunnel barriers to
two external leads having different chemical potentials $\mu_L$ and $\mu_R$.
The obtained results have been compared with other approaches existing in the
literature. They all agree in the equilibrium situation and the departures grow
with increasing the difference $\mu_L-\mu_R$.
