The theory of electrical double layer has 100 year history since Gouy and Chapman proposed. Since then the three-dimensional numerical solutions of Poisson-Boltzmann (PB) equation were widely applied for many systems such as chemically modified electrode or the electrode with micro-pore. However there are some shortcomings in PB theory for strongly electrified interface, ion-ion interaction in concentrated solution, and multivalent counter ions. In this article the canonical Monte Calro simulation with primitive model, which is simplest way to simulate the electrical double layer, is introduced.