In April 1986, large amount of radioactive pollutant was emitted into the atmosphere by the nuclear accident at Chernobyl, and dispersed around the world. Numerical simulation of the global scale dispersion of the radioactive pollutant was carried out by using the operational meteorological model of the Japan Meteorological Agency. Calculated concentrations in many places over the world agree well with those observed. The numerical model showed that the radioactivity reached Japan about seven days after the accident with the maximum concentration of about 10pCi/m³, in agreement with observations.