To improve the structural resistance of a floor panel by storing a floor receiving material in a floor beam, and inserting and fixing the projections protruded from the end sections of the floor panel between the inner face of the floor beam and the floor receiving material.

Multiple floor joists 14 each integrally formed with projections 20 at both end sections are provided in the gable direction, end floor joists 15 are provided at both end sections in the gable direction, and side floor joists are provided at both end sections in the girder direction to form a floor frame. A floor face material 17 and a lower face material 18 are fitted to the floor frame to form a floor panel 12. A floor receiving material 23 made of a lip shape steel is stored and fixed in a floor beam 13 made of U-shape steel. The floor beam 13 fixed with the floor receiving material 23 is fitted between the floor panels 12. The projections 20 of the floor joist 14 of the floor panel 12 are inserted between the upper flange 25 of the floor beam 13 and the floor receiving material 23. The floor beam 13 is fixed to the projections 20 by screws 26, and it is connected to the floor panel 12 to form a building unit. The rigidity of the floor beam 13 is increased, and the structural yield strength of the floor panel 12 can be improved.