The finite element method is applied to calculate the leakage magnetic field from an induction heating system consisting of an exciting coil and a conducting circular plate. The basic technique of the method is to draw a mathematical sphere to enclose the system. The total energy functional is assumed to be given by the interior and exterior functionals. The former is represented by using the usual axisymmetric triangular elements. On the other hand, the latter is given in a simple form by expanding exterior fields in terms of the solutions of a differential equation governing exterior empty spaces. The interior and exterior potentials are then matched on the spherical interface. After the validity of the method is verified by comparing results with analytic solutions for a single coil, the leakage magnetic flux from an induction heating system is computed at large distances.