We analyzed the chemical composition of porewater extracted from sediment samples collected at shallow methane hydrate fields off Joetsu, in the Japan Sea, during Expedition CK22-03 of the drilling vessel Chikyu in 2022. At Joetsu Knoll, where massive methane hydrates were observed, the Cl^– concentrations of porewaters were high (879–1282 mM) at the depth interval between 12 and 60 meters below the seafloor and roughly twice the concentration in the sediment at the top of the core (545 mM). These elevated Cl^– concentrations were observed at greater depths and were higher than those reported in previous studies of this field. We analyzed the δD and δ¹⁸O of the porewaters and quantitatively confirmed that the hypersaline waters represent residual waters accumulated by rapid formation of methane hydrates. Based on Cl^– concentration, amount of methane incorporated into hydrates in the sediments was estimated to be 51–81 m³/m³ at standard temperature and pressure.