In order to predict tritium concentration at ground level near a nuclear site, a conceivable process for tritium transfer in the natural ecosystem must be traced. We developed an Easy Evaluation System for Atmospheric Dispersion (EESAD) code based on the random walk method (RWM) for calculation of the atmospheric dispersion of tritium. The code can deal with the hourly change of weather conditions and tritium release rates as are mainly observed in an accidental release. In order to validate its prediction accuracy, and to verify its effectiveness, we calculated using scenario 3 (constant release) and scenario 4.2 (intermittent release)supplied by BIOMASS (Biosphere Modeling and Assessment) program by IAEA. Tritium concentrations predicted by EESAD calculation agreed well with those observed. Tritium deposition from the plume (dry and wet), re-emission from the soil surface, and infiltration to the lower soil layers were all considered in the EESAD system, and found to be effective to get better agreement. The EESAD is useful for calculating not only a controlled constant release with meteorological changes but also an instantaneous release with hourly changes of the release conditions.