We used push corers during manned submersible dives to take sediment samples to depths up to 40cm from the third and eighth Kumano Knolls. Boron (B) concentrations in pore water extracted from the sediment samples from the cold seep site were higher than would be explained by organic matter decomposition, suggesting that the pore water at the site was influenced by B derived from smectite-illite alteration. Additionally, the pore water at the cold seep site on the Kumano mud volcano showed high Li concentrations and positive oxygen isotope of pore water. These facts suggest the origin of the pore water would be clay mineral dehydration occurring between 150-160℃. The end-member concentration of B and Li is estimated to be 23±8mmol/kg and 0.8±0.3mmol/kg, respectively. The B/Li ratio of the end-member is evaluated to be 29±14, suggesting these elements would be released from sediment below 200℃. Given the geothermal gradient in this area, this finding suggests that the supplied fluid originates from environment deeper than 3.5km below the seafloor.