We report the strontium isotope ratios (87Sr/86Sr) from pore water collected during Integrated Ocean Drilling Program Expedition 337 off Shimokita peninsula, Japan. We use the ratios to track the origin of pore water between 1289 to 2405 meters below seafloor at Site C0020. Strontium isotope ratios measured from the samples collected by conventional sediment squeezer and the Quicksilver probe are in good agreement and suggest the success of the latter sampling technique. The 87Sr/86Sr ratios in the pore water range from 0.7086 to 0.7061, suggesting the mixing between ancient seawater and fluids derived from volcanic material alteration, a process also confirmed by onboard smear slide observations.