A free-standing rack system, in which each rack is not fixed to the floor or the wall, is proposed and is in use in European countries and the US as a storage method for spent fuel from nuclear power stations. Although this system can reduce the influence of an earthquake's excitation force by using the frictional force between the rack's bottom surface and the floor surface, together with the fluid force excited by each rack's motion, design guidelines are not yet established. In this research project, to evaluate the fluid force more precisely, the gap between the racks is treated as a two-dimensional gap flow, and the pressure loss coefficient at the flow path junction and the top of the flow path were estimated based on the steady CFD calculation and incorporated into the motion model. Our primary concern in this paper is rocking motion. As a result, we concluded that increasing the pressure loss coefficient at the fuel rack's top suppressed rocking motion.