Abstract: An ultrasonic flaw detector for immersion testing of articles, comprising a focusing planoconcave lens whose concave surface is conical, a piezoelectric plate of trapezoidal shape with electrodes applied on to the surface thereof, and an ultrasonic oscillation attenuation unit, all three elements contacting each other and being arranged in the order given so that the concave surface of the focusing planoconcave lens and the piezoelectric plate have a common main symmetry plane and the directrix of the concave surface of the focusing planoconcave lens is disposed at the side of the larger base of the piezoelectric plate.