Prehung door unit structure intended to close an opening, more particularly of rectangular or square shape, made in a stonework construction such as a wall, rising substantially vertically and separating or demarcating two spaces within a building and, more particularly, within a high-security building, said stonework construction comprising a concrete wall, or first-phase concrete, and a micro-concrete, or second-phase concrete, said structure comprising, on the one hand, a prehung door unit (3, 4, 5) made up of at least one door leaf mounted so that it can pivot, via the intermediary of at least two hinges, each equipped with a fixed portion and a movable portion, about a vertical axis, and of a fixed frame comprising two vertical uprights, a horizontal upper cross-member and, where appropriate, a threshold, said prehung door unit (3, 4, 5) being mounted in a rebate made in the thickness of the wall of the embrasure of the opening and, on the other hand, a fixed-frame pre-frame mounted in the rebate of said opening while being held and locked in position in said rebate, characterised in that the prehung door unit comprises, in addition to prefixing means, anchoring means (11', 11"'), sealing means (11") and rigid and removable connecting means (12, 13), and in that: - the fixed-frame pre-frame is, on the one hand, prefixed directly in said concrete wall or first-phase concrete via the prefixing means and, on the other hand, fixed rigidly and permanently in the stonework construction via the anchoring means (11', 11"') and the sealing means (11"), said sealing means (11") consisting of the micro-concrete or second-phase concrete, sunk into said fixed-frame pre-frame, said anchoring means extending into the micro-concrete or second-phase concrete, - said fixed-frame pre-frame constitutes a support for attaching the prehung door unit (3, 4, 5), so that said prehung door unit (3, 4, 5) can be fixed removably into said opening using rigid and removable connecting means (12, 13), said fixed-frame pre-frame comprising at least one receiving support piece (14, 15) and said prehung door unit (3, 4, 5) comprising at least one connecting piece (16, 17, 18) assembled with a receiving support piece (14, 15), by screwing one against the other under pressure via the rigid and removable connecting means (12, 13), - a housing or cavity (19, 20) is provided in each receiving support piece (14, 15) to receive the corresponding rigid and removable connecting means (12, 13). - the fixed-frame pre-frame is, on the one hand, prefixed directly in said concrete wall or first-phase concrete via the prefixing means and, on the other hand, fixed rigidly and permanently in the stonework construction via the anchoring means (11', 11"') and the sealing means (11"), said sealing means (11") consisting of the micro-concrete or second-phase concrete, sunk into said fixed-frame pre-frame, said anchoring means extending into the micro-concrete or second-phase concrete, - said fixed-frame pre-frame constitutes a support for attaching the prehung door unit (3, 4, 5), so that said prehung door unit (3, 4, 5) can be fixed removably into said opening using rigid and removable connecting means (12, 13), said fixed-frame pre-frame comprising at least one receiving support piece (14, 15) and said prehung door unit (3, 4, 5) comprising at least one connecting piece (16, 17, 18) assembled with a receiving support piece (14, 15), by screwing one against the other under pressure via the rigid and removable connecting means (12, 13), - a housing or cavity (19, 20) is provided in each receiving support piece (14, 15) to receive the corresponding rigid and removable connecting means (12, 13).