A sudden increase of stratospheric aerosols has been observed by a Nd-YAG lidar at the wavelengths 1.064 &mgr;m (F) and 0.532 &mgr;m (S) since June 4th, 1980 (all times JST = GMT+9). The increase is thought to be caused by the eruption of Mt. St. Helens in May, 1980. The initial increase began at an altitude of about 15km and the scattering ratio at wavelength F reached a maximum on June 5th. This indicates that the aerosol mixing ratio is about 70 times the non-volcanic ratio at the same altitude. The extinction of the vertically incident direct solar radiation has been calculated to be 2¿10-2 on the average for June and July, which is 4.6 and 3 times the non-volcanic value, respectively, for the fall of 1979. After examination of the size distribution of aerosols inferred from a comparison of profiles at the F ans S wavelengths and data by other measurements, it may be concluded that our lidar has seen relative temporal and spatial variations of the aerosols fairly well, but underestimated their absolute magnitude due to the assumption of clean air near 30km. A method for correcting the underestimation would increase the extinction by a factor of 1.4-2.0. This method is described and global distribution of aerosol inventory estimated on the basis of a two dimensional analysis.