The knowledge of atmospheric v's is becoming important, since the flux ratio, v(mu)v(e), observed with the water cherenkov type -detector shows a significant deviation from the predicted value. The uncertainty in the calculation of atmospheric 's are mainly originated from the uncertainties of the flux of primary cosmic rays and hadronic interactions. The absolute value of the cosmic ray flux is rather difficult to determine and the uncertainty is estimated ~15%, although the spectrum shape and the relative fraction of chemical components are well determined in <~ 100 GeV. Since available experiments are limited, some properties of hadronic interactions, such as the production spectrum of secondary particles, are not well determined either.

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