Evaluation of annual average of radionuclide concentration in surface air obtained from atmospheric dispersion factor is intended to determine a public dose as a primary source for the safety analysis of nuclear facilities in normal operation. Oarai Research and Development Center (ORDC) of the Japan Atomic Energy Agency have used fixed 5-year meteorological statistics for derivation of atmospheric dispersion factors as average conditions. To show that the meteorological statistics for any 5-year period could be used as representative data for evaluation of average conditions, annual average (1-year average) and 5-year average of evaluated radionuclide concentrations derived from the meteorological data observed over a 20-year period (1991-2010) at ORDC were analyzed. Fluctuations of evaluated radionuclide concentrations of any 5-year average were smaller than those of 1-year averages. Further, any 5-year averages were sufficiently convergent to 20-year average. Because any 5-year averages contained no rejections by the <i>F</i>-test (5% significance level), they were not statistically different to the rest of 20 years data set, instead that some of 1-year averages could be rejected. It means that any 5-year averages of radionuclide concentration evaluations are well representative for the safety analysis of normal operation of the nuclear facilities in ORDC.