The confidence of estimated Allan variance （?）y2（T0）is studied with the random signal processing theory.The signal processing models for calculating （?）y2（T0） and σ2[σy2（T0）] from y（t） are set up

and their calculation formulas are derived. The problem of increasing the confidence and decreasing the measurement time is discussed. The confidence interval of σy2（T0） is calculated for the given confidence coefficient. For white noise of frequency the above theory is verified by computer simulations.This paper points out that the formulas about σ2[（?）y2（T0）]/σy4（T0） in ref. [5] are approximations of the formula （13） in this paper under the specified condition

and some of the autocorrelation coefficients in ref.[6] are improper.