To construct a void slab having a uniform construction by placing concrete only once without jumping or waving of an embedded member for forming the void.

A bottom frame 6 forming the bottom surface of a slab 1 (void slab) is installed; reinforcements 2 used as bottom bars 2L of the slab 1 are arranged in a floated state from the bottom frame 6; an embedded member 3 made of a foamed resin is arranged in a floated stated from the bottom bar 2L and fixed without moving up-and-down, back and forth and right and left relative to the bottom frame 6; a bar 2 used as a top bar 2U of the slab 1 is arranged in a state floated from the embedded member 3; and the slab 1 is constructed by placing concrete 5 to a predetermined height in which the top bar 2U can be fully embedded.