Abstract: For the inspection of a square billet, an ultrasonic testing technique using an angle beam is performed using a phased array system. The phased array probe is disposed in a plane perpendicular to the axial direction of a square billet, at a prescribed distance from the surface of the billet and set at a prescribed angle with respect to the billet surface. The billet is inspected both inside and at the surface layer when the ultrasonic beam is electronically scanned. Inspection of the surface layer only is also performed using a surface defect inspection apparatus. Determination of internal defects (including subsurface defects) is performed based on the subtraction of information from the surface defect inspection from the information from the ultrasonic inspection.