  There have been a lot of works concerning the Strichartz estimates for the
perturbed Schr\"odinger equation by potential. These can be basically carried
out adopting the well-known procedure for obtaining the Strichartz estimates
from the weighted $L^2$ resolvent estimates for the Laplacian. In this paper we
handle the Strichartz estimates without relying on the resolvent estimates.
This enables us to consider various potential classes such as the
Morrey-Campanato classes.
