A novel tokamak with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is much reduced to a theoretical limit determined by the virial theorem is designed and constructed. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest stress requires equal averaged principal stresses in all directions. Therefore, the pitch number of a helical coil is determined to satisfy the uniform stress condition. Moreover, the helical winding is modulated in such a way that poloidal field exists only outside of a torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. In the case of the aspect ratio A = 2, our optimal coil theoretically increases the magnetic field to 1.4 times larger than the conventional TF coil.