The chloride anion is known to be the most abundant salt ion in sea water. At the regions such as ODP Sites 1249 and 1250 the highly enriched chloride concentration is observed in a zone extended from near the sediment surface (~1 mbsf) to depths about 25 mbsf. Here, we designed the in-situ electric circuit system for measuring chloride concentration within reliable accuracy. In the cylindrical cell the 5-10 tubes having holes on the wall and electrodes were equipped around clay mixture. The open holes were made to regulate to a certain degree the interface area between methane gas and clay sample. As may be anticipated, the chloride concentration abnormally increased under fast rate condition for forming methane hydrate, but no noticeable concentration change was detected under relatively low rate. In fact, the present experiment seems to be a lot deficient to investigate the ion diffusion and moreover does not fully reflect the real deep-sea floor condition, but the meaningful results for describing the abnormal salinity enrichment might be drawn. The physical effects of chloride anions on surface morphologies of methane hydrate formed in the sediments were additionally examined with the Field Emission-Scanning Electronic Microscope (FE-SEM).