To manufacture a concrete floor slab at a low cost.

Bottom plates 2 are placed on the flanges 1b, 1b, 1b, 5b of adjoining reinforcing materials 1 and side plates 5, they are integrally fitted together by an adhesive or the like, and reinforcement 3 is inserted into through holes 1d of the webs 1a of the reinforcing materials 1, 1. Ready-mixed concrete C is placed on the bottom plates 2 to solidfy it integrally with them. Consequently, this concrete floor slab A is constituted so that the bottom plates 2 functioning as a form are left integrated with the concrete C and the like as they are, and hence a mold releasing work becomes unnecessary. The flanges 1b, 5b of the reinforcing materials 1, 5 reinforce their strength, in addition they function to support the bottom plates 2, hence the bottom plates 2 are easily assembled to execute the manufacturing work skillfully and quickly, and the cost can be reduced.