A local alignment algorithm finds a substring pair of given two strings where two substrings of the pair are similar to each other. A DNA sequence can consist of not only A, C, G, and T but also N and X where N and X are used when the original bases lose their information for various reasons. In this paper, we present an efficient local alignment algorithm for two DNA sequences including N and X using the affine gap penalty metric. Our algorithm is an extended version of the Kim-Park algorithm and can be extended in case of including other characters which have similar properties to N and X.