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We have measured the 3 He/ 4 He and 20 Ne/ 4 He ratios of gases dissolved in the pore water in sediments at two sites in the Nankai Trough (Site 583) and the Japan Trench (Site 584). The 3 He/ 4 He and 20 Ne/ 4 He ratios vary from 0.215 × 10~6 to 1.23 × 10~6 and from 50 × 10^3 to 2700 × 10~3, respectively. These values can be explained by mixing two components, one from the atmosphere and one with a 3 He/ 4 He ratio of (0.2 to 0.3) × 10~6. The latter component may be derived from the ocean crust near the subduction zone.

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