Integrated Ocean Drilling Program (IODP) Expedition 331 was conducted in September 2010 and provided an unique opportunity to directly access the subseafloor environment beneath an active hydrothermal field in the Iheya North Knoll in the mid-Okinawa Trough. Localities of five drilled sites (C0013-C0017) along the slope of a knoll cover the hydrothermal field, represented by vigorous high temperature fluid venting from the active hydrothermal mound named NBC. Drilling at Site C0016 at the flank part of the mound revealed hydrothermal sulfide/sulfate mineralization comparable to that in Kuroko-type volcanic massive sulfide deposit. From Sites C0013 and C0014 located 100 and 450 m east from the mound, evidence for lateral intrusion of the hydrothermal component was observed by intense and widespread hydrothermal alteration, stepwise change in profiles of pore fluid chemistry, and drastic change in physical properties of the sediment. On the other hand, downward infiltration of seawater was recognized even at 50 m below the seafloor at Site C0017 located 1550 m apart from the mound. These results suggest the mixing region between the high-temperature and reductive hydrothermal environment and the low-temperature and oxidative seawater-dominant environment outspreads over a km scale beneath the active hydrothermal field. Furthermore, microbiological studies revealed that habitat of subvent microorganism significantly overlaps this mixing region. Expedition 331 was successful in providing a new model for a hydrothermal fluid circulation system that develops in a geologic setting dominated by volcanic sediment.