This paper discusses a Busemann type supersonic biplane aiming at designing supersonic biplane aircrafts with a low sonic boom. It is shown that a rectangular supersonic biplane in lifting condition does not make pressure disturbance due to its volume around the biplane in the supersonic area rule when the three-dimensional effect near the wing tips are neglected. To confirm this result in fully three-dimensional nonlinear flows, this paper presents a method to decompose equivalent area distributions, which are related to pressure disturbance, into their components. With this method, Computational Fluid Dynamics (CFD) and the multipole analysis are employed to show the advantage of the supersonic biplane in a lifting condition in fully three-dimensional nonlinear flows. Although the equivalent area of the supersonic biplanes due to their volume does not vanish completely, it is considerably smaller than that of monoplanes. Because a total equivalent area distribution is constrained by Seebass-George-Darden (SGD) sonic boom minimization theory, a resulting small equivalent area due to wing volume would allow a wider fuselage. Finally, a design of a practical supersonic biplane wing considering takeoff, landing, and climb requirements is shown.