PURPOSE: To detect a flaw in a weld between a straight pipe and an elbow by arrang ing two probes so that a horizontal isosceles triangle whose base line is composed of the line connecting the probes to each other and vortex is at the intersection of the perpendicular bisector of the base line and a surface to be inspected can be formed.

CONSTITUTION: A transmitting probe 1 and a receiving probe 2 are arranged so that, a transmitting beam LT and a receiving beam LR can be propagated through the vicinity of the internal surface of a section to be inspected in nearly the same line when the beams LT and LR are viewed from the side face in the axial direction. Consequently, both the beam LT from the probe 1 and the beam LR of reflected echoes from a defect F are propagated in parallel with the internal surface of a plate in the longitudinal section of the section to be inspected at the position where the defect exists and the noise of reflected echoes, etc., from penetration beads can be reduced remarkably, because the defect can be detected without making ultrasonic waves to incident to the penetration beads. Therefore, since the noise of reflected echoes, etc., from the penetration beads of the weld of an austenitic stainless steel pipe are remarkably reduced, the defect can be detected with high accuracy through a weld metal.