Abstract: An object of the invention is to obtain dislocation preventing means which has a simple structure and reliably prevents dislocation from concrete, and cause a deck plate to correspond to a resistance cross section with respect to all dead/live loads over the entire length of a bridge beam. A dislocation preventing bolt 9 has a threaded section 91, having an external thread formed thereon, a tapered section 92, which is formed continuously from the threaded section, and a circular cylindrical section 93, which is formed continuously from the tapered section. A longitudinal rib composite floor slab 100 includes a main girder 1, a transverse girder 2 joined to the main girder 1, a deck plate 4 whose lower surface side is joined to the main girder 1 and the transverse girder 2, a longitudinal rib joined to the upper surface of the deck plate 4, concrete 7 placed on the upper surface of the deck plate 4, and pavement 5 placed on the upper surface of the concrete 7.