The three-dimensional P-wave velocity structure of the Iwate volcano, northeastern Japan, is determined to depths of 2 km through an active seismic survey conducted in October 2000. Seismic tomography is applied to approximately 2700 travel-time data. The most prominent discovery is an existing of column-like high-velocity body (Vp > 5.4 km/s) that extends vertically for 2 km beneath the caldera. While the western part of the volcano extending from the caldera is characterized by a moderate-velocity region (4.8 < Vp < 5.4 km/s), the summit and eastern flank of the volcano are covered with very low-velocity material (Vp < 4 km/s) which represent relatively younger volcanic edifices. The spatial difference in the velocity structures between the western and eastern parts of the volcano is explained by the evolutionary history of the volcano. And we find that the western structure may give constraints on the volcanic activity in 1998.