Water lubrication was investigated with a double-network hydrogel (DN-gel) using a pin-on-disk tribometer. The friction coefficient obtained in water was 0.025. For IR measurement, a diamond prism of attenuated total reflectance (ATR) unit was in contact directly with DN-gel and the sliding experiments were carried out in water. IR spectra at the sliding interface were obtained by an evanescent wave with a Fourier transform infrared-attenuated total reflectance (FTIR-ATR) unit during lubrication tests. The concentration of water and gel at the interface was estimated from the IR absorbance of OH and CH stretching modes. It was found that the water content at the interface increased by sliding compared to a static condition and did not change with sliding speed but decreased with increasing load. In order to confirm the water source at the interface, the lubrication test was carried out in heavy water (D<inf>2</inf>O) as a tracer. The water in the gel was not exchanged with D<inf>2</inf>O. It can be concluded from the spectral data that the water at the sliding interface was mainly originated from bulk water (D<inf>2</inf>O).