Providing a half precast slab even in a case where the thickness of the slab in the site condition is too thick to apply conventional precast construction techniques. A half precast slab including a plurality of floor concrete form members and, each floor concrete form member being provided with: a bottom slab of a long length, in response to the floor concrete form member; a pair of side walls installed upright along each longitudinal edge of the bottom slab, in response to the floor concrete form member; thereby, each floor concrete form member is arranged so as to be parallel to each other and be tightly connected to the adjacent floor concrete form member, along the direction perpendicular to the longitudinal direction of each floor concrete form member; the height of the side walls other than the side walls on the most outer edge sides of the whole floor concrete form members is set at a common height that is smaller than the height of the side walls on the most outer edge sides.