We investigate the shell evolution in unstable nuclei with large-scale shell-model calculations. Since the tensor force has been pointed out to be promising in accounting for the shell evolution, we construct a new interaction in the sd-pf shell region whose tensor force in the cross-shell part is determined with attention. The resulting strength is much stronger than the Millener-Kurath (MK) interaction which has been often used as the cross-shell interaction, giving rise to a significant difference from that by MK in the proton shells in the N = 28 region and the nuclear structure of 42Si.