[en] The axial electric field of a positive column in a low pressure discharge tube has been obtained from the energy balance equation of electrons, in which the energy gain of electrons from the axial electric field balances with the energy loss by the collision with gas atoms and the diffusion to the wall. However, the theoretical results have been different from the measurement in lower gas pressure range. Its main reason is considered to be due to the energy loss by the radial electric field. In this paper, we have analyzed this energy loss theoretically on the basis of the theories by Schottky, Tonks and Langmuir and Dote respectively. (author)