Abstract: Herein disclosed is a method for ultrasonic flaw detection of T-welded portion of steel product. The method comprises: maintaining the flange portion horizontally, making convergent ultrasonic beams incident on the flange portion vertically to the top face of the flange portion through a medium, scanning the incident beams transversely of the welded portion, detecting ultrasonic echo from a depth corresponding to the bottom face thickness of the flange portion, and comparing the level of said ultrasonic echo with a predetermined threshold value. Disclosed also is an apparatus for carry out the above method. These method and apparatus are suited particularly to the on-line flaw detection of the welded shape steel.