This article reviews geochemical studies on submarine hydrothermal activity conducted by the author these 20 years. It is well known that submarine hydrothermal activity plays an important role in chemical budget between the lithosphere and the ocean, affecting chemical components of seawater and biogeochemical cycles in the ocean. The author's first target was the mid-Okinawa Trough, northwestern Pacific Ocean, where high temperature hydrothermal fluids (220 to 320℃) are erupting from sediment-covered arc and back-arc systems. In order to collect hot fluid samples using the submersibles Shinkai 2000 and Shinkai 6500, the author and his coworkers developed a deep pump sampling system with a multi-port rotary switching valve. The author's studies were extended to submarine hydrothermal systems in the western Pacific back-arc basins, such as the north Fiji Basin, Mariana Trough, and Manus Basin (Papua New Guinea) as well as the Okinawa Trough, where unique characteristics of hydrothermal fluids have been clarified using CH₄, Mn, Fe, Al etc. as hydrothermal tracers. A highlight was that Shinkai 6500 discovered strongly acidic hydrothermal fluids (pH<2) in the eastern Manus Basin, which is possibly affected by incorporation of a magmatic fluid and disproportionation of the exsolved SO₂ from a magma body below the seafloor. The author further extended his study field to mid-oceanic ridge systems, particularly that at the Rodriguez Triple Junction which connects three ridges of the Indian Ocean. Three cruises were devoted to the triple junction area. First, extensive surveys using a CTD-hydrocast system were performed during the R/V Hakuho Maru cruise in 1993, successfully finding hydrothermal plumes to speculate their origin to be at a topographic high called Hakuho Knoll in the eastern off-axis zone from spatial distributions of light transmission and CH₄/Mn ratio. Then Shinkai 6500 dove twice at the knoll in 1998, confirming the existence of hydrothermal activity. Finally, the R/V Kairei cruise with ROV Kaiko dives in 2000 found a hydrothermal active field with black smoker chimneys (Kairei Field) for the first time in the Indian Ocean, at 2,450 m depth along the southwestern slope of the knoll. Chemical and isotopic characteristics of the 360℃ fluid were found to be similar on the whole to those so far observed at mid-oceanic ridges in the Pacific and the Atlantic Oceans. For future studies, it is noted that the δ¹³C (CH₄) is a new useful tracer characterizing microbial activity in hydrothermal plumes.