To explain quantitatively the lubricating effect of gas molecules on adhesive wear, the authors have introduced a new concept of mean free time of a small contact point. When a bridge formed between two contact points is sheared off and a small clean surface is exposed to surrounding gases, it must have a free time t until it forms the next bridge when sliding. During this time t, the gas molecules attack the clean spot and can easily form a chemisorbed monolayer when the contact pressure p is low or when the sliding speed v is low. The authors have verified the fundamental relation between the mean free time t and the value pv, and have found that the critical transition point from the mild to severe wear corresponds to the condition where the mean free time of a sheared junction is just enough to be covered by the chemisorbed monolayer of surrounding gas molecules.