Apparatus for testing the seal of an explosion-proof box, such box provided with an inner surface, delimiting an inner space of said box, and with a peripheral edge, delimiting an access opening to said inner space; said apparatus comprising: - at least one closing body, which is provided with a resting surface on which the peripheral edge of said box is intended to be arranged in a sealing relationship; - at least one insertion body, which is fixed to said closing body, is provided with an outer surface, and it is adapted to be inserted in the inner space of said box through the access opening of the latter with said outer surface of said insertion body turned towards the inner surface of said box; said apparatus being characterized in that it further comprises: - at least one membrane, which is arranged as a coating of at least the outer surface of said insertion body and delimits, with the outer surface of said insertion body, at least one compression chamber; said membrane being adapted to delimit with the inner surface of said box at least one air gap; - first feeding means connected to said compression chamber by means of at least one first passage opening obtained on said closing body, and adapted to introduce at least one pressurized working fluid in said compression chamber to push said membrane to act under pressure on the inner surface of said box; - evacuation means connected to said compression chamber by means of said first passage opening of said closing body, and adapted to evacuate said at least one working fluid from said compression chamber; - second feeding means connected to said air gap by means of at least one second passage opening obtained on said closing body, and adapted to introduce at least one working fluid in said air gap to be interposed between said membrane and the inner surface of said box. - at least one closing body, which is provided with a resting surface on which the peripheral edge of said box is intended to be arranged in a sealing relationship; - at least one insertion body, which is fixed to said closing body, is provided with an outer surface, and it is adapted to be inserted in the inner space of said box through the access opening of the latter with said outer surface of said insertion body turned towards the inner surface of said box; - at least one membrane, which is arranged as a coating of at least the outer surface of said insertion body and delimits, with the outer surface of said insertion body, at least one compression chamber; said membrane being adapted to delimit with the inner surface of said box at least one air gap; - first feeding means connected to said compression chamber by means of at least one first passage opening obtained on said closing body, and adapted to introduce at least one pressurized working fluid in said compression chamber to push said membrane to act under pressure on the inner surface of said box; - evacuation means connected to said compression chamber by means of said first passage opening of said closing body, and adapted to evacuate said at least one working fluid from said compression chamber; - second feeding means connected to said air gap by means of at least one second passage opening obtained on said closing body, and adapted to introduce at least one working fluid in said air gap to be interposed between said membrane and the inner surface of said box.