Abstract: This invention concerns a building module for producing flat construction, especially walls, which has two parallel, panel-shaped wall parts (2, 3) the exterior surfaces of which form the wall to be constructed. A module core (14) of wood connects the two wall parts (2, 3). The module core (14) has at least one module core part (5, 39, 47, 71) which runs in the longitudinal direction (X) of the building module. When an additional building module is put in plate between the wall parts (2, 3), this core (14) protrudes into this additional building module in the vertical direction (Z) and forms with these wall parts (2, 3) an interlocking connection which takes up forces in the transverse direction (Y). Means (19, 20; 77, 78) are provided for the purpose of forming an interlocking connection which takes up forces in the longitudinal direction (X) when an additional building module is put into place.