A comparison of trans-Pacific hydrographic observations revealed changes in properties of North Pacific Intermediate Water (NPIW) in the subtropical gyre of the North Pacific from 1980s and 1990s to 2000s. Salinities decreased on density surfaces above the NPIW salinity minimum and increased below the salinity minimum. The potential temperature of the NPIW core increased by 0.10 -- 0.17¿C, although the salinity of NPIW core did not change substantially. The pattern of salinity changes on neutral surfaces was consistent with a density decrease of the NPIW core. It is suggested that the property changes of the NPIW core can be explained by NPIW warming.