Four ball tests were carried out with lubricant oil of PAO (poly-α-olefin, kinematic viscosity: 394 mm²/s) without additives under the atmosphere of deuterium gas. It was found that deuterium was detected on the friction track of lubricated upper-ball by TOF-SIMS(Time of Flight-Secondary Ion Mass Spectrometry). The intensity ratio of observed deuterium/hydrogen (D/H) to the natural abundance of D/H (R(D/H)) is used as a parameter for the relative intensity of deuterium on the surface. The obtained R(D/H) of the friction track was 50. The cross-section of the upper-ball was also analyzed with TOF-SIMS. After 2 nm of top surface layer was removed by means of ion etching, the surface of 50 μm square was analyzed from 0 to 300 μm of depth from the uppermost surface of friction track. It was found that the R(D/H) decreased gradually with depth and became to the natural abundance at the center of the ball. The result clearly shows that deuterium gas, D₂ was decomposed tribochemically and D was diffused from the friction surface to the bulk under lubricating condition.