To provide a lightweight placing-laying type double floor with a wiring gutter, which is made of steel, which can be easily constructed, and which can be adapted to unevenness.

Floor slabs 3 are regularly laid and arranged on a floor surface at prescribed intervals, and a parallel wiring gutter 51 is formed between edges of floor boards. A cross-shaped crossed wiring groove 52 is formed in a place wherein the parallel wiring grooves cross each other. Placing edges 14 are formed on both sides along the wiring grooves 51 and 52, and a rectangular lid and an octagonal lid are detachably placed. The floor slab is supported by many supporting legs which are formed by downward bending and/or bulging a peripheral extension part of a rigid thin plate for forming a surface of the floor board.