In a road bridge, drainage facilities are installed to discharge the water on the bridge surface. For each drainage facility, installation spacings are exemplified by waterproof manuals and others, but the quantitative sharing and drainage capacity of each facility and bridge surface have not been clarified. Also, it is thought that water will be stagnant due to bridge surface irregularities generated during construction or during service. However, no regulations on the concavities and convexities about the slab surface are clearly shown in construction standards. Therefore, in this study, surface shape measurement and water sprinkling test on unpaved actual bridge slab were carried out. From the results, we confirmed the possibility that irregularities on the slab surface affect the tendency of local water stagnation. Since only one bridge was studied in the current study, we have not confirmed the influence of gradient magnitude on the drainage capacity of uneven slab surface. It is a future task to clarify the relation between surface gradient and allowable irregularity size and to develop a standard for controlling irregularities on slab and pavement surface.