The size distribution function of stratospheric aerosol layer is discussed based on the data by laser radar at Fukuoka and the small ion concentrations deduced from the results of radiosonde observations at Kagoshima. If we use a size distribution spectrum essentially similar to that originally proposed by Friedlander (1960), both data are explained consistently. It is shown that the obtained mass concentrations are sensitive functions of assumed refractive indices, and the order of magnitude agrees well with the aircraft sampling result. The relaxation time is examined on the above model by using gas kinetic theory in the subrange, where Knudsen number is greater than 3. It is shown that the Aitken particle concentration near the lower end of the spectrum is numerous and the rate of coagulation is adequate for the in situ formation theory of aerosols and the relaxation time of the SO2 gas oxidation required by the analysis of variations of the isotope concentration ratio <32S>/<34S> due to Castleman (1970). The influence of the volcanic injection into the stratosphere on the modification of the size distribution is briefly examined, and resulting variations of parameters are discussed.