Numerical calculations were performed to simulate charging characteristics of an electron beam printhead. Because the electric conduction in the gas discharge field is determined not only by electrostatic potential but also by ionic or electronic charge density, two coupled partial differential equations, the continuity equation of charge and Poisson's equation, were numerically solved by the finiteelement method under the boundary condition that the electric field at a discharge electrode is a constant determined by a corona starting electric field strength. It was demonstrated that the calculated portion of the high charge density corresponds to the observed luminous and highly degraded portion. The calculation also confirmed that predominant charging particles on the printhead are not ions but electrons. The present method can be utilized for practical design of the charging process.