On 31 March 2000, the Usu volcano located in Hokkaido, Japan began erupting after 22 years of dormancy. After the first phreato-magmatic explosion, there were many small-scale phreatic explosions at the Nishiyama and Kompirayama Craters. In April and May, observations were made using a piezo blast-sensor and a visible video recorder to get a better understanding of volcanic explosions. The data thus obtained were analyzed using the scaling-laws that relate surface phenomena to both explosion energy and depth. The analysis using the shape of the explosion-cloud and the peak over-pressure of the volcanic blast-wave indicated that the average explosion energy and depth of the phreatic explosions of mid-April ranged from 1010 to 1012 J and originated from between just beneath the ground surface to a depth of a few tens of meters.