In order to improve the riding quality of Shinkansen, test runs in the speed range of 70∼210 km/h were carried out on Joetsu Shinkansen, April 1982, where many kinds of track irregularity were set artificially. Regression analysis was done to confirm the quantitative relation between car vibrations and running speed in each track irregularity section. As a result, it was made clear that the lateral car-body vibration grew remarkably on lateral alignment irregularity sections of 30∼50 m wave-length in the speed range over 150 km/h. Further, it became possible to estimate amplitudes of each irregularity which harmed the riding quality, and it was confirmed that a suitable change in the damping coefficient of lateral oil damper was effective for reduction of the lateral vibration.