Checking device (200; 300), for checking the quality of folding boxes, each folding box comprising, when the folding box is in a folded state, at least two flaps separated by a slot, the slot having a peripheral end which leads to a peripheral face of the folding box and which delimits a lateral interval between the two flaps, the checking device (200; 300) comprising: - an illumination system (202; 302) configured to illuminate the folding boxes in the folded state, and which is configured to illuminate a pack formed of superposed or juxtaposed folding boxes, said pack having at least one peripheral pack face defined by peripheral faces of each folding box composing the pack, and - at least one camera device (204; 304) configured to form images of the folding boxes in the folded state, the checking device (200; 300) being characterised in that: - the illumination system (202; 302) comprises two illumination sources (202.1; 202.2; 302.1; 302.2) arranged on either side of said at least one camera device (204; 304), the illumination sources (202.1, 202.2; 302.1, 302.2) being configured to successively illuminate one same peripheral face of each folding box in the folded state such that said at least one camera device (204; 304) successively forms two different images of the same peripheral face of each folding box in the folded state, - said at least one camera device (204; 304) is configured to form at least one plural image representing at least one portion of the peripheral pack face when the illumination system illuminates the peripheral pack face, and in that it further comprises at least one image processing system (206; 306) configured: i) to detect each peripheral end in the plural image, ii) to generate a set of data representative of the geometry of each peripheral end, and iii) to analyse the set of data so as to determine the width of each lateral interval. - an illumination system (202; 302) configured to illuminate the folding boxes in the folded state, and which is configured to illuminate a pack formed of superposed or juxtaposed folding boxes, said pack having at least one peripheral pack face defined by peripheral faces of each folding box composing the pack, and - at least one camera device (204; 304) configured to form images of the folding boxes in the folded state, - the illumination system (202; 302) comprises two illumination sources (202.1; 202.2; 302.1; 302.2) arranged on either side of said at least one camera device (204; 304), the illumination sources (202.1, 202.2; 302.1, 302.2) being configured to successively illuminate one same peripheral face of each folding box in the folded state such that said at least one camera device (204; 304) successively forms two different images of the same peripheral face of each folding box in the folded state, - said at least one camera device (204; 304) is configured to form at least one plural image representing at least one portion of the peripheral pack face when the illumination system illuminates the peripheral pack face, i) to detect each peripheral end in the plural image, ii) to generate a set of data representative of the geometry of each peripheral end, and iii) to analyse the set of data so as to determine the width of each lateral interval.