We propose a new method applying the diffusionconvection model for numerical simulation of toner concentration distribution so as to design development unit with stable toner concentration at the development area. Numerical calculation by the discrete element method using damper and spring model for individual particle is usual to analyze dynamic behavior of each particle, however it needs enormous calculation time because of the great number of particle to be treated and to account the interaction of them in development unit. To decrease the load of calculation, simple experimental data are introduced beforehand. As the result, knowing the diffusion coefficient and the convection speed of developer in a development unit, the toner concentration distribution can be estimated which shows good agreement with experimental results, and an actual machine can carry out appropriate toner concentration control by adopting this method.