PURPOSE: To provide an ultrasonic flaw detection method and device suitable for utilizing in detecting inner defects such as non-metal medium inside a rolled metal plate such as a steel plate.

CONSTITUTION: An ultrasonic sensor 20 with transmission type constitution of a line focus type ultrasonic sensor 58 and a one dimensional ultrasonic array sensor 59 are arranged face to face putting a thin steel plate 12 in between. By transmitting a belt shape ultrasonic wave beam 23 from the line focus type ultrasonic sensor 58 nearly vertically to the thin steel plate 12, receiving the ultrasonic wave having penetrated through the thin steel plate 12 with the one dimensional ultrasonic array sensor 59 and extracting only the penetrated wave 24, the existence of penetrated wave 24 at or below a specific amplitude is detected. In this manner, an inner defect near the surface can be detected without dead zone and in a line region of a certain width at once.