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Conspicuous anomalies in gas compositions were observed at a fumarole and three mineral springs about 1–3 months prior to an inland earthquake of 6.8 magnitude in central Japan, on September 14, 1984. The epicentral distances are 9 km for the fumarole and 50, 71, and 95 km for the springs. The anomalies in He/Ar, N2/Ar, and CH4/Ar ratios of bubble gases from the springs can be attributed to fluctuations in the emission rate of some deep‐seated gas due to change in pore pressure, which is subjected to crustal stress, resulting in the earthquake. The anomaly at the fumarole showed an increase in discharge rate of deep volcanic gas under a compressional stress near the focal region. Since the behavior of subsurface fluids represented by He/Ar and other ratios is presumably controlled by the change of pore pressure, temporal variations in the fluids can be used as a strain gauge for the crust. On the other hand, the precursory emissions of H2, which were observed after changes in He/Ar and other ratios had occurred, may have resulted from a reaction between groundwater and the surface of newly formed cracks. The timing and amplitude of the geochemical anomalies are compatible with the result predicted by a dilatancy model without fluid diffusion, although H2 emission may be related to groundwater permeation into newly formed cracks. Monitoring of several gases with automatic gas chromatographs at several stations may be useful for unraveling earthquake mechanisms and for predicting earthquakes.

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