Recently, bootstrap currents, generated spontaneously in a tokamak plasma, have been observed in large tokamak devices. Those bootstrap currents are applied to designs of a next-generation experimental tokamak reactor like ITER and following tokamak power reactors. A simple expression for a bootstrap current of a tokamak with a large aspect ratio is proposed, which can easily evaluate the bootstrap current. Some empirical scalings for a bootstrap current with a finite aspect ratio have been proposed so far, and the applicability of them is briefly discussed. (J.P.N.).