To provide concrete floor-slab structure, in which required strength characteristics can be ensured without particularly increasing slab thickness even when a floor lowering section is installed in an intermediate section between girders supporting the end sections of a floor slab respectively.

A floor lowering section Sw is mounted to an intermediate section between a pair of girders BY1, BY2 supporting the end sections of floor slabs F1-F4 and F5-F8 respectively in the concrete floor-slab structure. Beams Bs made of precast concrete are extended along the stepped section of the floor lowering section Sw, and the floor slabs F1-F4 and F5-F8 made of precast concrete are arranged on both sides of the beams Bs respectively at that time.