The present invention relates to a lubricant for lubricating relatively movable, facing contact surfaces at least one of which is coated with DLC, a method of lubricating DLC contact surfaces with this lubricant, and a system having DLC contact surfaces. The lubricant fulfills the following conditions (a) and (b): (a) the lubricant contains a lubricant base oil containing, as a main component, a base oil composed at least one of a hydrocracked mineral oil, a wax-isomerized mineral oil, and a poly-+--olefin base oil. The base oil has a kinematic viscosity of 2 to 20 mm 2 /s at 100 DEG C, a total aromatic content of not higher than 5 wt%, and a sulfur content of 0.005 wt%; and (b) the lubricant has a sulfur content of not higher than 0.2 wt%.