Provided is an apparatus by which a nucleic acid may be analyzed quickly and precisely. Specifically, provided is an apparatus for analysis of a nucleic acid including: a processing unit for creating, based on a total base composition of a nucleic acid, a plurality of base composition sets, creating a hierarchical structure in which the base composition sets are hierarchized in ascending order of a number of bases in a partial base composition at a terminal position, and further creating connection relations between each of the base composition sets, and another base composition set having, at the terminal position, a partial base composition obtained by adding one base to a partial base composition at the terminal position of each of the base composition sets; a processing unit for calculating, for each of the partial base compositions, a predicted mass value of a corresponding product ion, and imparting a weight to each of the base composition sets based on a comparison between the predicted mass value and an actual mass value of a product ion derived from the nucleic acid; and a processing unit for selecting one of base composition sets belonging to each hierarchy while following the connection relations from an outermost hierarchy in the hierarchical structure, based on a weight imparted to each of the base composition sets, to thereby determine a base sequence sequentially from a terminal base of the nucleic acid.