In order to make it possible in ultrasonic flaw detection to generate ultrasonic waves containing a main beam only by use of an array probe and clearly identify a defect in a specimen by use of images, an element pitch P (the distance between centers of adjacent ultrasonic transducer elements in the array probe) is set longer than ¼ of the wavelength of longitudinal waves generated by the ultrasonic transducer elements and shorter than ½ of the wavelength and reception signals up to time corresponding to the sum of wall thickness round-trip propagation time for longitudinal waves and wall thickness round-trip propagation time for shear waves in the specimen are displayed.