Over pressure of a volcanic blast has been measured for the first time by a lead-plate blastmeter at Unzen Volcano, Kyushu, Japan. The over pressure of the volcanic blast on June 8 was 0.28 bar at 2700 m distance from the crater, and that was less than 0.06 bar at 4400 m distance. The estimated explosive yield of the explosion accompanied by the blast was 1.1¿107 kg TNT (5¿1020 erg) calculated based on the over pressure and distribution of the blast effect. The explosion energy is as much as 1/600 of that of the blast at Mount St. Helens on 18 May 1980. The lead-plate of the blastmeter at 2700 m has also recorded the collision with some accretionary lapillis. The maximum particle velocity of the blast was estimated to be about 75 m/sec based on the trace of the collision. The significance of over pressure for the prediction of volcanic hazards is also discussed briefly. ¿ American Geophysical Union 1993