To improve workability when a concrete slab is constructed by engaging plate-like members with a plurality of engaging recessed parts formed on foaming plastic plates with each other at the engaging recessed parts to form a well-crib-liked, timbering and installing a formwork on the timbering.

A plate-like member 10 is formed with a plurality of engaging recessed parts 11 on the one side at fixed intervals. By forming the engaging recessed parts 11 of depth (h) to be nearly a half of the height H of the plate- like member 10, a plurality of the plate-like member of the same form can be assembled into a well-crib form of which the upper face and the lower face are flush. It is desirable that the width (w) of the engaging recessed part 11 is formed a little smaller than the thickness (t) of the plate-like member 10, and hence the well-crib in the extremely strong condition can be assembled. The height H of the plate-like member 10 is properly decided according to the height from a working floor to a slab formwork. The thickness (t) and the assembling pitch of the plate-like member 10 are decided considering the height H of the plate-like member 10, the thickness of a slab, and the like.