A sheet processing apparatus 3 includes a position aligning section (61A, 61B), a holding section 53, and a processing section (38, 55). The position aligning section (61A, 61B) abuts the inner side or a fold of a sheet bundle (S2), which is formed by collecting a plurality of folded sheets (S1), to align the position of the folds of line plurality of folded sheets (S1). The holding section (53) holds the sheet bundle (S2) in which the position of the folds of the plurality of sheets (S1) has been aligned by the position aligning section (61A, 61B). The processing section (38, 55) performs a predetermined process on the sheet bundle (S2) held by the holding section 53.