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 . <EOS>