According to one embodiment, a gain adjustment unit executes gain adjustment for 3D B-mode data using gain adjustment values respectively decided for the depth on each scanning line, azimuth direction and elevation direction in a scanned region. A threshold decision unit decides a threshold used for discriminating lumen and non-lumen regions in the scanned region using the gain-adjustment-executed 3D B-mode data. A threshold processing unit executes threshold processing to discriminate data concerning the non-lumen region from the gain-adjustment-executed 3D B-mode data, using the threshold. An image generation unit generates an ultrasonic image concerning the lumen region based on the threshold-processing-executed 3D B-mode data.