A water-resistant closing device for a watch case, the device including an annular gasket (1) arranged to be placed between a middle (2) and a back (3), a bezel or a crystal (4), or a bezel and a crystal, or a back and a crystal. The gasket (1) has on its middle portion a bulge (10) protruding on its outer lateral face (11), the bulge being arranged to be engaged against the middle (3) and compressed following the installation of the back or of the crystal on the middle, the bulge being delimited by an upper (12) and lower material countersink (13) on either side of the bulge (10) in the non-compressed state of the gasket (1) to facilitate its installation and its deformation.

A watch head (1) which can be assembled from the top and disassembled from the bottom, the watch head (1), including: a case (2) having a middle (3) delimiting an inner bed (3a), the back (4) fitted with reversible fastener on the middle (3) and a movement rest (5) mounted flush with a lower face on the middle (3); a movement (7) mounted on the movement rest (5) within the pre-assembled case (2); a dial (8) mounted on the movement (5); and a glass (10) closing the pre-assembled case (2) at the end opposite to the back (4). The dial (8) and the movement (7) have dimensions less than or equal to the dimensions of the bed (3a) inside the middle (3) so that the middle (3) can be traversed from above and from below.

A watchcase (1) including a middle (3) suitable for holding a movement (8) and a cell (9) slotted in a compartment of the movement (8), a back (2) assembled with a bayonet connector in the middle (3) and a bolt (6) fitted on the bayonet connector to lock any relative rotation between the back (2) and the middle (3).