To obtain excellent strength by a method wherein a plurality of hollow pipes between girders in the horizontal direction, main reinforcements are arranged at the vertical places of the hollow pipes while disposing distributing bars and shear reinforcements are latched to the upper and lower distributing bars in the vertical direction.

An upper-end main reinforcement 2 and a lower-end main reinforcement 3 are arranged in parallel with a hollow pipe 1 having an isosceles triangular cross section on the top face and underside of the hollow pipe 1, and distributing bars 4, 5 are disposed in the rectangular direction to the hollow pipe 1. Shear reinforcements 6, 7 are latched and welded along the inclined side faces of the hollow pipe 1 so that an angle of inclination γ to perpendicularity is set from 30° to 60° in the upper and lower distributing bars 4, 5, and concrete 8 is placed by a concrete pump, etc. Accordingly, truss structure is assembled to void slab structure and strength is increased, and its own weight of a slab is lightened and cost can be reduced.