Some bridge piers in a megacity such as Tokyo have been constructed by steel in the 1960s, because of the very strict construction limit. Most of the steel piers were welded at that time, when weld has been started to apply from the riveting. However, little attention was paid for the weld quality. The quality of some of the welded joints was not satisfactory. That caused fatigue cracking, especially at the intersection of three weld lines that can be seen in a steel pier. Insufficient bevel processing resulted in an incomplete penetration. In case of intersection of three lines, the incomplete penetration forms a volumetric defect. This study aims at the detection of incomplete penetrations. Recently, phased array ultrasonic technique has been applied because of the higher steering ability such as focusing and/or changing angles. A specimen of the beam column joint of almost the same dimension as the actual structure was made. A volumetric incomplete penetration was made in the intersection, and planar incomplete penetrations were made in the beam-column flange joints by intentionally changing the bevel and root gap. The planar incomplete penetration could be detected by the phased array ultrasonic test by the use of acrylic wedge because relatively large angle of refraction was necessary. The three dimensional incomplete penetration at the intersection was assumed to be consisted of several surfaces in a triangular prismatic shape. Indications could be obtained from some surfaces. In contrast, it was very difficult to obtain indications from some other surfaces due to the accessibility. However, the volumetric incomplete could be detected, and the size of the surfaces could be estimated based on the information from the accessible side and the assembly of the plates. Phased array ultrasonic technique enhanced the detection ability comparing with the conventional ultrasonic test by the focusing characteristics.