Chemical and isotopic compositions of brines collected from the Minami-Kanto natural gas field in Chiba, central Japan, were measured to investigate their origin and relation to microbial methane formed from the coexisting brines. The isotopic compositions of water in the brines are -3.8 to -1.5‰ and -15 to -1‰ in δ¹⁸O_H2O and δD_H2O values, respectively, which are correlated with their chloride concentrations, suggesting that the brines are derived from seawater and that some of them have been mixed with meteoric water. The brines have high concentrations of iodine up to 0.14 g/l which are related to those of dissolved bromine and ammonia. These dissolved chemicals are suggested to have been liberated from marine sediments into pore water through the diagenetic decompositions of organic matter. The hydrogen isotopic compositions of methane (δD_CH4) in the natural gases are -195 to -146‰. The average isotopic difference between δD_CH4 and δD_H2O for the samples is about 180‰, suggesting that the methane was formed by microbial activity via a CO₂-reduction pathway. However, because the data obtained in this study were widely scattered on the δD_CH4-δD_H2O plot, it is not possible to confirm that the obtained data follow the linear correlation predicted by Schoell (1980) for methane formation by CO₂-reduction.