In February 2002, a subsurface mesoscale eddy with extraordinary features was detected at a depth of about 300 dbar in the southeast of Okinawa (a part of the Ryukyu Islands). The water mass occupying this subsurface eddy is characterized as the North Pacific Subtropical Mode Water (NPSTMW) with salinity maximum and thick thermostad; its temperature, salinity and potential density at the core of the eddy are 18.4¿C, 34.81 and 25.06 σ$theta$, respectively. Thickness and width of the eddy are about 300 m and 100 km, respectively. Mixing between the eddy and the surrounding water mass is also found. Such eddy with the NPSTMW has never been detected during ten years.