  We demonstrate how the Exact WKB analysis works in the study of the
quasi-normal modes (QNMs). We apply the Exact WKB analysis to a massless scalar
perturbation to the D3-brane metric as a concrete example. The Exact WKB
analysis provides an exact condition for the QNMs. We numerically check our
exact condition by using the Borel-Pad\'{e} approximation. We also present an
analytic form and an asymptotic behavior of the QNMs.
