[en] Along the general scheme of Sondheimer and Wilson, the kinetic energy density of an electron gas under constant magnetic field is expressed as a functional of the electron density at absolute zero of temperature. On this basis, the statistical theory for atoms in a magnetic field is formulated, which includes the theory developed by Banerjee et al. as an extreme of high magnetic field. Some numerical results on the atomic radius, the total energy etc. are also shown for free neutral Ne atom. (author)