The protective box comprises a base (2) and a lid (3), these being designed to fit together. The box also comprises a side plate (6) inserted into the base and held laterally by the rim (4), and an elastically deformable block (7) designed to immobilize a plate-like part pierced by an opening (11). The block is mounted on the upper face of the side plate and is designed to be inserted into the opening of the plate-like part. The lower face of the lid (3) also has a projection (8) designed to press against the block when the box is closed, so as to increase the outside diameter of the block and immobilize it in the opening of the plate-like part. The elastic means (9) are also provided to move the side plate (6) back towards the lid (3) in order to keep the block (7) immobilized in the opening of the plate-like part.