We collected pore waters using an in situ pore water-squeezer for a submersible Shinkai 2000 at six depths beneath the sediment surface within a deep-sea “cold seep” giant clam community off Hatsushima Island, Sagami Bay, Japan. A box core sample was also collected ca. 4.5 km east of the community and pore waters were separated. Dissolved inorganic carbon (DIC) was extracted and purified in a vacuum line and 14C concentration was determined with a Tandetron accelerator mass spectrometer at Nagoya University after conversion to graphite targets using a batch Fe-catalytic hydrogen reduction method. ∆14C values decreased with increasing depth to −938‰ at the sulfate concentration minimum. This indicates that methane used for the active reduction of sulfate and formation of hydrogen sulfide, which is used by symbiotic chemoautotrophic bacteria in gills of the giant clams, is almost dead and is likely supplied from the deep. ∆14C values of DIC vary linearly with δ13C values along a mixing line between that in the bottom water and that produced by the oxidation of dead methane. The δ13C value of DIC oxidized from dead methane is estimated to be ca. −45‰.