In order to study the latent image formation process including the characteristics of laser beam and photoreceptor, a numerical method which solves the three dimensional time dependent coupled system of the transport equations for charges, and the Poisson equation for electrostatic field, is proposed. Combining the simulation with electrostatic field computation in the development zone, the strength and the extent of spreading of the electrostatic field due to the latent image is analyzed. The image of isolated one dot exposure shows larger spreading, while that of a dot alongside another dot shows weaker electrostatic-field strength. The dependence of the spreading on the beam radius indicates that the repulsion of charge due to space-charge effect becomes significant as the beam diameter decreases. The numerical results are compared with experimental ones obtained on a test bed with high resolution development, and show excellent correlation with the experimentally developed images.