Multiple penetration heat flow measurements were made using Ewing type and violin bow type instruments on the cruise of R/V SONNE in 1984. The studied areas are the Ryukyu Trench between the Kyushu-Palau Ridge and the Amami Plateau, the northernmost part of the Okinawa Trough, and the middle Okinawa Trough around 127 degree E. In the Ryukyu Trench area, the heat flow is 50 to 65 mW/m**2 near the trench axis and it seems to be lower on the landward wall. The heat flow could not be determined in the northern Okinawa Trough, because the temperature profiles were very non-linear, probably due to variations in the bottom water temperature. In the middle Okinawa Trough, extremely high heat flow values were observed in a depression called the Natsushima-84 Deep, located in the central rift region. Combined with the results of the DELP-84 WAKASHIO cruise, the average of heat flow in the depression is about 500 plus or minus 400 mW/m**2. The very high and variable heat flow, as well as non-linear temperature profiles, suggest the existence of hydrothermal activity.