PURPOSE: To make it possible to drain surplus water contained in cast-in- placeconcrete in construction of a floor structural body using a deck plate, at the same time, to drain the surplus water efficiently within a short period of time to prevent cracks in the cast-in-place-concrete, to make it possible to increase strength of the concrete and to reduce a construction period of time.

CONSTITUTION: Single or a plurality of corrugated sheet-like deck plates 1 used as the bottom section 4 of each of recessed grooves 3 and providing a plurality of small holes 5 passing through the deck plate lengthwise at specific intervals is laid horizontally on a floor beam. After a belt-shaped filter cloth 2 having permeability against water and air and having non-permeability against cement particles is additionally provided on the upper surface of the bottom section of each of the recessed grooves so as to cover the small holes, concrete 9 is placed on the deck plate, and it is cured and hardened to construct a floor structural body.