To investigate a fine structure of the 3He/4He ratio distribution in and around a single volcano, fumarolic gases, geothermal gases and waters were collected at 13 sites within the caldera of the Hakone volcano, Japan. The 3He/4He and 4He/20Ne ratios vary from 6.7 × 10-6 to 9.6 × 10-6, and 0.85 to 26, respectively. The 3He/4He ratios, corrected for air contamination, of samples within the caldera are uniform and as high as those of magmatic fluids in the Japanese Islands. The 3He/4He ratios of two geothermal water samples outside the caldera are significantly lower than those of inside. The observed 3He/4He ratio distribution is interpreted as an indication of the presence of source magma with a high 3He/4He ratio beneath the volcanic region.