In this paper, we propose a new transcranial direct current stimulation (tDCS) system which uses multiple electrode arrays. Once a target location inside a human brain is selected, optimal injection current at each electrode is determined by solving a constrained optimization problem, of which the objective function is evaluated by 3D finite element method (FEM) adopting the superposition principle for enhanced computational efficiency. Simulation studies demonstrated that the proposed tDCS system enables more effective and accurate focusing on targeted brain areas than the conventional system.