Epiphytic bryophytes and lichens are valid indicator plants of air pollution. High correlation between pollution concentration and an index calculated from the species composition, coverage and frequency of such plants has received much recognition. This index is called IAP (Index of Atmospheric Purity). We have successfully made a computer program intended for use with Lotus1-2-3 to give IAP, which may make this index more useful owing to the following three advantages: 1. making it possible to calculate IAP by using as only input the coverage (%) of species obtained at each study station; 2. requiring shorter calculation time; 3. if neccessary, allowing recalculation to be effected simply by addition of the input data for new study stations which must be further taken into account.