The invention relates to an apparatus for the one-piece production of a room module (8) having three side elements (24) and a floor element (25) and/or a ceiling element (26), wherein the apparatus takes the form of a parallelepipedal and hollow core formwork (10) around which an outer formwork (9) for the room module (8) to be produced can be arranged. The following are provided here: a) a rectangular head element (11) which consists of first planar elements (1) and second planar elements (2), b) side elements (12, 13, 14, 15) which consist of a third planar elements (3) and fourth planar elements (4) and are arranged perpendicular to the head element (11) in the edge regions thereof, wherein at least the third planar elements (3) or the fourth planar elements (4) have corner regions (17) of the side elements (12, 13, 14, 15), c) the first planar elements (1) can be lowered under the second planar elements (2) of the head element (11) and can be displaced with respect to one another and, subsequently, the second planar elements (2) can be pushed together with a reduction of the rectangular area of the head element (11), and d) the third planar elements (3) and fourth planar elements (4), which are arranged perpendicular to the head element (11), of the side elements (12, 13, 14, 15) can then be moved in succession in the direction of a centre longitudinal axis (16) of the head element (11) or the core formwork (10).