A fastening device for folding boxes, the folding boxes including a bottom and four lateral walls coupled in jointed fashion to the edges of that bottom, two opposite walls having each fastening devices for ensuring the unfolding of the box, the device in general including a vertically displaceable central piece and some lateral bolts with horizontal displacement for fastening the lateral walls via their adjacent edges. The folding device includes elastic spring elements which link the central control piece with the pairs of bolts, at the same time, the spring elements, in the rest position, maintain the bolts in the engagement and fastening position of the bolts, while for the release position of the bolts the spring elements pull the bolts towards the release position against the resistance of the elastic elements.