The unstacker device (1) for unstacking flat articles comprises: a magazine (3) in which the articles (2) to be unstacked are disposed on edge in a stack against a jogging edge (5) and facing an unstacking plate (4) arranged to separate a first article (2A) and to drive it in a certain unstacking direction (A) that is perpendicular to the jogging edge (5); a conveyor (7) for conveying the unstacked articles, the conveyor having an inlet (7A) downstream from the jogging edge (5); and retaining means (8) disposed between the jogging edge (5) and the inlet of the conveyor (7A) and actuated to exert a retaining force on an article (2A, 2B), which retaining force opposes movement of the article. The device further comprises a detector device (9, 10) suitable for sensing and analyzing the trace of the article moving between the jogging edge and the conveyor so as to detect the presence of a plurality of articles moving together, and, on the basis of such detection, so as to actuate said retaining means for the purpose of separating said articles.