A distinct hydrothermal plume was found in the Iheya Graben in the middle Okinawa Trough in May, 1987, by using methane, manganese and the helium isotope ratio (3He/4He) as the geochemical tracers. The plume rises to an altitude of about 300-400 m above the sea floor with maximum methane and manganese anomalies of up to 650 nl kg-1, and 0.54μg kg-1, respectively. The 3He/4He ratio in the center of the plume is 1.65 times that of atmospheric helium. These anomalies are significantly higher than those previously observed for the shimmering water from the hydrothermal mounds discovered in the same area by the submersible “SHINKAI 2000” in July, 1986. This strongly suggests that a more active hydrothermal system should exist nearby the hydrothermal mounds. Together with previous geophysical and geological studies, the present results further support the view that the middle Okinawa Trough is in an active stage of back-arc spreading behind the Ryukyu trench.