A composite beam structure used in the construction industry, in which one or more slab-like pieces (2) and a beam (3) are connected together, advantageously with a casting (4), and which composite beam structure includes at least one beam, such as a steel beam, on top of which is fastened one or more dowel pieces (4), and in which the length of the dowel pieces fastened in the beam is at least threefold in relation to the width of the upper surface of the dowel pieces, and the dowel piece is able to transfer forces in the connection between concrete and steel both in the vertical and longitudinal direction of the beam.