Preparedness of emergency evacuation for the leakage of toxic substances in chemical plants is very important to reduce damages. In order to implement emergency evacuation properly, it is necessary to consider the assumption of leakages, atmospheric conditions and prediction of consequences of dispersed gases comprehensively and concretely, and repeated training for emergency is also important. As concrete studies, early detection of toxic leakage is necessary, and a study of effective installation of gas detectors in the manufacturing area and at the plant boundary is carried out. After detection of the leakage, it is necessary to obtain information such as leakage rate, wind speed, wind direction, atmospheric stability, and it is also necessary to predict the dispersion area. The prediction of the area must be carried out easily and anyone can obtain the same result. The prediction program is developed in this study by excel for easy to use and distribute many workers. Finally, for appropriate decision making of evacuation, it is necessary to decide which threshold value of toxic gas should be taken. The comprehensive method for emergency evacuation is developed and discussed in this study.