The proposed magnetic circuit precisely evaluates magnetic fluxes between the field magnets of dual Halbach arrays. In the case that an actuator is configured with the dual Halback arrays and the armature coils between them, the air gap length between the dual Halbach arrays gives the maximum number of interlinkage magnetic fluxes with the armature coil through algebraic approach. The new design method is on the basis of the proposed magnetic circuit. This method makes it clear that the air gap length should be more than 1.2 times longer than the permanent magnet poles length. The utility of this design method is verified through designing cylindrical linear synchronous motor provided with the dual Halbach arrays.