Device with a laser window fitted on an outer wall of an enclosed process chamber containing a modified atmosphere for layered construction of a 3D object by means of a laser, where this outer wall features at least an opening that is fitted with a laser window and where this laser window lets a laser beam from a laser source located outside of the process chamber pass through the opening and into this process chamber to build the 3D object in layers, where an external sealing ring is provided that extends between the outer wall and the laser window, wherein the improvement comprises that the laser window is movable with respect to the opening, where, moreover, a sealing plate is provided that extends into the process chamber and that is movable between an open position and a closed position, where, in the open position of the sealing plate, this sealing plate leaves the opening in the outer wall clear, and where in the closed position of the sealing plate: - an internal sealing ring is provided that extends into the process chamber between the sealing plate and the outer wall, around the opening, - the sealing plate, combined with the internal sealing ring, seals the opening of the process chamber, - around the opening, a lock chamber is created that is bordered by at least the sealing plate, the internal sealing ring and the laser window, - an inlet and an outlet extend into the lock chamber for supply of the modified atmosphere in this lock chamber. where this outer wall features at least an opening that is fitted with a laser window and where this laser window lets a laser beam from a laser source located outside of the process chamber pass through the opening and into this process chamber to build the 3D object in layers, where an external sealing ring is provided that extends between the outer wall and the laser window, wherein the improvement comprises that the laser window is movable with respect to the opening, where, moreover, a sealing plate is provided that extends into the process chamber and that is movable between an open position and a closed position, where, in the open position of the sealing plate, this sealing plate leaves the opening in the outer wall clear, and where in the closed position of the sealing plate: - an internal sealing ring is provided that extends into the process chamber between the sealing plate and the outer wall, around the opening, - the sealing plate, combined with the internal sealing ring, seals the opening of the process chamber, - around the opening, a lock chamber is created that is bordered by at least the sealing plate, the internal sealing ring and the laser window, - an inlet and an outlet extend into the lock chamber for supply of the modified atmosphere in this lock chamber. - an internal sealing ring is provided that extends into the process chamber between the sealing plate and the outer wall, around the opening, - the sealing plate, combined with the internal sealing ring, seals the opening of the process chamber, - around the opening, a lock chamber is created that is bordered by at least the sealing plate, the internal sealing ring and the laser window, - an inlet and an outlet extend into the lock chamber for supply of the modified atmosphere in this lock chamber.