A double floor structure capable of being adapted to the conditions of construction and the needs of users at low cost. A double floor structure (K) provided with support legs (1) which are provided on a lower floor and rows of beams which form an upper floor. The support legs (1) are each provided with an upper member (14) which supports a beam from the lower side, an intermediate member (13) which supports the upper member (14) from the lower side, and a lower member (12) which supports the intermediate member (13) from the lower side. The upper member (14), the intermediate member (13), and the lower member (12) consist of metallic, extruded shape material and are disposed in such a manner that the direction of the extrusion is aligned with the top-bottom direction.