|In the conventional design method, there is no particular method to judge the liquefaction phenomenon of the unusual soil such as volcanic ash soil. This study, in order to clarify and evaluate the liquefaction characteristics of volcanic ash soil, field investigations and a series of soil laboratory tests of volcanic ash ground were carried out. The field investigations and sampling were conducted in the areas where ground liquefaction occurred due to the 1993 Hokkaido-nansei-oki and the 2013 Tokachi-oki earthquakes. According to the results, the liquefaction strength ratio (RL) was underestimated when using the formulation of the conventional design method. Moreover, it was found that the estimated formulation RL need to be modified for evaluating the liquefaction phenomenon in volcanic ash soil.