The chemical compositions of gas samples from the steaming ground of the Komatsu Jigoku in the Kuju geothermal area, Kyushu, Japan, were measured. Using a gas geothermometer based on H
2, CO
2, CH
4 and H
2S contents (D'Amore and Panichi, 1980), the underground temperatures of the Komatsu Jigoku were calculated. From the relationships between the obtained temperatures and the CO
2/H
2 and CH
4/H
2 ratios, two gas geothermometers excluding H
2S contents are developed. The equations are presented as follows, TCO
2/H
2 (°C) = 352.45 - 53.36 log (CO
2/H
2)TCH
4/H
2 (°C) = 228.95 - 24.06 (CH
4/H
2). These gas geothermometers were applied to gas samples from the production wells of the Hatchobaru and the Otake geothermal areas, Kyushu, Japan. The obtained temperatures (TCO
2/H
2 and TCH
4/H
2) agree with those (Tquartz) estimated by silica content in the geothermal waters within 20°C in a temperature range between 200°C and 250°C. Besides, the bubble gases from hot springs having different water temperatures (97°C and 25°C) gave reasonable temperatures (195°C to 209°C) in the Otake production area. Thus, it can be concluded that these simple gas geothermometers are useful for estimating the temperatures of geothermal systems (<250°C). Furthermore, CO
2/H
2 geothermometer is found to be successfully applicable to the temperature beyond 250°C. On the other hand, the values of TCH
4/H
2 become relatively lower than those of TCO
2/H
2 and Tquartz above 250°C. The reason of lowering is not clear at present. Further test of the CH
4/H
2 geothermometer at other geothermal systems is required to investigate the reason.
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