Fishkova Ioffe Physicotechnical Institute of the RAS, 26 Polytechnic str., St. Petersburg, 194021, Russia E-mail: fishkova@mail.ru An electrostatic system to be used for monochromatisation of an electron beam has been proposed. <...> It consists of the basic filter on energy in the form of the flat capacitor with the closed ends in a mirror operating mode and the additional capacitor compensating the initial energy spread. <...> The analytical formula of the field strengths relationship has been obtained. <...> Calculation of this system has shown that the electron beam energy spread has become about 10 times less than the initial thermal spread. <...> Она состоит из основного фильтра по энергии в виде плоского зеркала с закрытыми торцами и дополнительного плоского конденсатора, компенсирующего начальный разброс по энергии. <...>