PURPOSE: To enhance construction properties by forming a reference floor into a polygon on plan view, and forming all floor boards by the assembling of rectangular, flat small precast concrete panels, in addition to using a plurality of triangular flat precast concrete panels only at the steps of connections between the sides of the polygon.

CONSTITUTION: A driveway 21 and a parking area 22 are set on the same plane within a large floor board 3, the area 22 installed in a direction perpendicular to the driveway 21. Thereby the surfaces of traveling and parking roads are flattened, improving a driving feel. Since the large floor board 3 is formed by the combination of small precast concrete panels according to the size of the parking garage, design, fabrication and construction are facilitated. Opposite upper and lower sides are connected together by large floor boards 3B, 3C to provide a driveway for descent only, thereby enhancing safety. Further, gradients at both side edges along the direction of extension of the driveway are made equal at each floor board 3, and the large floor board 3 is supported by vertical poles and horizontal beams to simplify a steel structure. Therefore, construction work can be carried out easily and accurately, and cost can be reduced.