Precipitation chemistry within Japan has been assessed in order to judge both the magnitudes of the concentrations and the deposition of major ions: H+, NH4+, Ca2+, K+, Mg2+, Na+, SO42, NO3−, and Cl−. A data set was obtained through an acid precipitation survey by Japan Environment Agency at 14 stations from April 1984 to March 1986. The annual average pH at each site ranged from 4.5 to 5.1 with a mean value of 4.7. The free acidity was derived both from sulfuric and nitric acids; sulfuric acid was found to contribute to 67–83% of the free acidity. The acidity was apparently lower than that expected from SO42− and NO3− levels, mainly because alkaline species, such as NH3, and Ca2+ species neutralized a certain fraction of the original acidity. This neutralization would certify the fact that SO42− and NO3− deposition were comparable to, or smaller than those in eastern North America, but the Ca2+ and NH4+ deposition within Japan exceeded that within the U. S.