[en] Method for solution of conjugated equation of neutron transport by the Monte Carlo method with detailed account of energy dependence of neutron reaction cross sections is presented. Fast neutron fluxes for three groups, coming from water barriers of different thickness have been calculated. The method enables to calculate neutron fluxes at any attenuation with practically available accuracy. Limits are imposed only by a word length of particular computer. Agreement of calculated data with those obtained by authors on the basis of group approach has been obtained