In recent years, several observations based on direct current measurements and model simulations have been successful in detecting a north east ward under current, “the Ryukyu Current”, along the Pacific side of the Ryukyu Islands with a unique “subsurface north east ward current core” structure. The volume transport (20 - 25 Sv) of the Ryukyu current completes the volume transport budget (45 - 50 Sv) of the Kuroshio system. A Pacific Ocean circulation model based on the RIAM Ocean Model (RIAMOM) with 1/12° horizontal resolution successfully reproduced the observed structures of the northeastward Ryukyu Current with a subsurface core at 500 - 600 m. A three-layer model simulation shows the existence of the Ryukyu Current and explains the mechanism of subsurface current maximum through blocking effect of bottom topography around the Ryukyu Islands.