PURPOSE: To obtain the depth of a drilled hole in good accuracy, by oscillating an ultrasonic wave in a drill, which drills a hole in a multi-layered vessel, and detecting a peculiar reflected wave generated when the drill is positioned in the boundary between a layered material and an inner cylinder.

CONSTITUTION: A drill 1 provides in its rear end a probe 5 brought into contact through oil, and an ultrasonic inspector 6 is connected with said probe 5. In this way, an ultrasonic wave, being reflected when the point end of the drill 1 passes through each layer of a layered material 8a, is detected as a fine G echo. Similarly an H echo is detected simultaneously when the point end of the drill 1 reaches a boundary position between the layered material 8a and an inner cylinder 7. When this H echo is detected, a motor 4 is stopped. Further the method shows an E echo for an oscillation wave from the ultrasonic inspector 6 and an F echo for an echo from the point end of the drill 1 while the G echo for an echo reflected from a clearance between the layered material 8a. In this way, the drill 1 drills a detection hole 9 in a correct depth to a predetermined boundary between the layered material 8a and the inner cylinder 7.