[en] Design of a central region is carried out for a compact superconducting AVF cyclotron for proton. The superconducting cyclotron has a strong magnetic field which makes the central region smaller. After construction, therefore, the degree of freedom of adjustment is restricted within narrow limits. For high-precision design, the three-dimensional calculation is required at the central region. A three-dimensional electric field produced by the central equipment, i.e. dees, ion source and reinforcement electrodes, is calculated by a relaxation method. The equation of particle motion in electromagnetic field is numerically solved by using the three-dimensional Runge-Kutta-Gill method. With sufficient energy gain, projection of trajectory on median plane is found to clear all electrodes and ion source. The vertical displacement is suppressed by electric focusing. (author)