A frame structure for casting a concrete building panel has a plurality of panel edge molding frame members with a generally U-shaped cross-section, a plurality of panel corner frame members with a configuration which corresponds to the angle of the corner of the panel. A number of edge molding frame members and the corner frame members are assembled in a panel molding frame with a length and width corresponding to the length and width of the panel to be molded with the ends of the edge molding frame members connected by the panel corner frame members, and with the opening of the U-shaped cross-section facing inwardly of the panel molding frame. Connectors rigidly connect the edge molding frame members to each other and to the panel corner frame members and a liner structure is provided in the interior of each of the edge molding frame members. The surfaces of the liner structure facing the space within the panel molding frame has a shape complementary to the shape of the edge of the panel to be molded.