Earth-atmosphere evolution models are mathematically simulated and the resulting present isotopic ratio ( 40Ar/ 36Ar) in the mantle is given for each. Differential outgassing experiments on several recent submarine glasses were made to estimate an isotopic ratio ( 40Ar/ 36Ar) in the present mantle. Estimations of ( 40Ar/ 36Ar) in the mantle by various methods are also critically reviewed. From the experimental results and these considerations a minimum value of 2000 for ( 40Ar/ 36Ar) ratio in the present mantle is inferred. By assuming that ( 40Ar/ 36Ar) M is larger than 2000 and that the potassium content in the present mantle is larger than 50 ppm, we can limit considerably a choice among various Earth-atmosphere evolution models, i.e. (1) a continuous degassing process can not explain rare gas evolution in the atmosphere, (2) early sudden degassing is more likely and (3) such sudden degassing must have occurred earlier than 4.35 b.y.