In estimating a structural formula of an unknown substance produced through partial structural change of an original substance having a known structure caused by metabolism or the like, structural change is considered in two stages, the elimination of a partial structure and the addition of another partial structure. First, an additional partial structure is collected as known information in addition to an MSn spectrum of the unknown substance and a structural formula of the original substance. A structural formula at the time when a partial structure is eliminated from the original substance is estimated, and a structural formula of each of product ions is estimated. The structural formula of the unknown substance is determined by estimating a structure that can produce the candidates for structural formulas of the product ions by dissociation.