The quasistatic magnetic field penetration through a circular aperture of a spherical shield enclosing a spherical conductor is considered in the framework of the dual series equations theory. Both cases of an applied magnetic field parallel and perpendicular to the system’s symmetry axis are analyzed. Induced currents on the cavity and internal sphere surfaces are calculated, as well as the aperture effective dipole moment and polarizability. Convergence of the different series involved is discussed. The magnetic field behavior at the aperture edge is also investigated, and the discontinuity occurring in this region is treated analytically. The theoretical analysis is finally complemented and illustrated by several numerical results.