Dielectric permittivity and loss are measured under steady shear flow as functions of temperature, shear rate, electric field frequency, and electric field strength in the nematic (N) and the isotropic (I) phases of 4-n-pentyl-4'-cyanobiphenyl. In the N phase, the dielectric permittivity in the quiescent state is largely modified if the steady shear flow is applied. These behaviors are discussed based on the Leslie-Ericksen theory [Q. J. Mech. Appl. Math. 19, 357 (1966); Arch. Ration. Mech. Anal. 4, 231 (1960)], showing that the dielectric properties under the shear flow are consistently interpreted in terms of the flow alignment of the director, a unit vector specifying the orientation of the liquid crystals. It is also suggested that the behaviors of dielectric permittivities are similar to those of the viscosities.