PURPOSE: To avoid the damage of an ultrasonic vibrator by avoiding the generation of heat stress or strain with a simple structure.

CONSTITUTION: An ultrasonic flaw detector is equipped with a reinforcing plate 1 made of SUS 304 steel having a hole part 1a in the, vicinity of the central part thereof, the ultrasonic vibrator 2 arranged in the hole part 1a of the reinforcing plate 1, the Cu substrate film 3 bonded to the reinforcing plate 1 and the ultrasonic vibrator 1, the Ni seal film 4 bonded to the main surface of the substrate film 3 and a seal box 5 surrounding the ultrasonic vibrator along with the reinforcing plate 1 and the substrate film 3 to cut off the same from the outside environment.