Bloch electron conductivity perpendicular to the layers of a superlattice (period d) is evaluated using an extension of the balance-equation qpproach [X.L. Lei and C.S. Ting. Phys. Rev. B 32, 1112(1985)] to narrow-band transport. The perpendicular park drift velocity Tp and The critical field Ec, at which the drift velocity peaks, are analyzed as functions of miniband width. Our theoretical predication that Ecd increases with decreasing miniband width agrees well with the data of Sibille et al. [Phys. Rev. Lett. 64, 52(1990)], even for the samples of narrowest miniband width in their expriment.