This study examines some topographic effects on the island rule. We use an idealized and barotropic model to investigate the throughflow between a semienclosed marginal sea and a larger oceanic basin that are connected to each other by two channels. Two sets of experiments are conducted in parallel, one with a flat bottom and the other with a ridge between two basins. The model results show that the ridge affects the island rule considerably in several ways. First, the ridge blocks geostrophic contours and restricts a free exchange between two basins. The bottom pressure torque (or the form drag) is a dominant term in the balance of the depth-integrated vorticity budget and always results in a significant reduction of the throughflow transport. Second, horizontal friction promotes cross-isobathic flows and enhances the throughflow transport over the ridge. This is the opposite of what friction does in the original island rule in which a friction tends to reduce the throughflow transport. Third, the forcing region in the open ocean for the marginal-sea throughflow is shifted meridionally. Last, the topographic effect becomes small near the equator due to its dependence on f. This may explain why the PV barrier effect is smaller in the South China Sea than in the Japan/East Sea. The limitation of the barotropic model and some baroclinic effects will be discussed.