Clinical observations of CO2-hot spring (CO2 ≥1000 ppm) immersion revealed the effects, an immersed part reddening, skin blood flow improvements, heart rate reduction3) , etc. The previous study was indicated that CO2-water immersion activates parasympathetic nerve activity in humans2). It is well known that not only systemic nervous system but also autonomous one are involved in posture control in quiet-standing position. CO2-water immersion may also influence the posture control through the autonomic nervous system activities. This study aimed to reveal the effect of CO2-water immersion on postural sway during quietly standing in healthy young adults. The healthy male college students (n=5) participated in this study. Body sway was evaluated by recording horizontal ground reaction force for 1 min continuously with a force platform equipped with a data processor. The recording was performed at pre-water immersion and post-water immersion under eyes open condition (EO) and then eyes closed condition (EC). Spectrum analysis by fast Fourier transform (FFT) method of body sway in the medio-lateral and antero-posterior axes was performed. Power spectra of body sway were evaluated by comparing powers in tow frequency bands: 0.02-1.0 (BAND 1), 1.0-10.0 Hz (BAND 2). Each subject was immersed up to the nipples for 15 min in tap and CO2-water (35°C), respectively. Cutaneous blood flow (BF) was measured on the upper chest and on abdomen by laser-Doppler flowmetry. Statistical evaluation of the data was done by repeated-measures two-way ANOVA, using Turkey test for post hoc multiple comparisons at the 0.05 level of significance. BF was significantly larger in CO2-water immersion throughout the recording period compared to tap-water immersion. Tap-water immersion changed neither path length of nor area of COP trace measured both under EO and EC conditions. However, the path length of COP was significantly decreased at post CO2-water immersion in EO (66.1±11.8 vs 55.5±15.7 cm, p<0.05) and EC (88.9±21.8 vs 76.9±17.7 cm, p<0.05). Body sway of BAND 1 was significantly decreased at post CO2-water immersion in the medio-lateral axes of EO (1.41±0.15 vs 1.13±0.26 cm2, p<0.05) and the antero-posterior axes of EC (1.72±0.19 vs 1.49±0.33 cm2, p<0.05), while that of BAND2 was not affected. It is known that body sway of BAND 1 is predominantly stabilized by visual input and vestibular input from the otoliths, and body sway of BAND 2 is stabilized by somatosensory input from the ankles and feet1). Present results indicate that CO2-water immersion can stabilize standing posture in human. Considering a decrease in BAND1 component, this may indicate a facilitation of visual input or vestibular input by CO2-water immersion.