During a big flood in recent years, flood waters intruded into the staircase between ground and underground in an urbane area. Aerated flows are formed in the staircase. Drag force acts on bodies or human legs in a high-speed-aerated flow. The authors proposed a method for estimating the velocity and the depth on stepped channels. Flow resistance and total head in stepped channels were clarified under a wide range of angle of slope, step height, and drop height. On the basis of the flow characteristics of aerated flows, the expression for a drag coefficient on bodies in a high-speed-aerated flow can be reasonably presented. Also, the drag coefficient of a model leg is obtained using the previous results.