An ultrasonic testing method for a weld zone, comprising: an arrangement step of arranging an ultrasonic probe (1, 1A, 1B) so as to face a weld zone of a test object, the ultrasonic probe (1, 1A, 1B) having n (n ≥ 2) transducers (11-1,..., 11-16) arranged along a direction orthogonal to a weld line of the weld zone of the test object; and a flaw detection step of selecting m (n > m ≥ 1) transducers from the n transducers (11-1,..., 11-16), and causing the selected transducers to transmit ultrasonic waves to the weld zone and to receive an echo from the weld zone, thereby detecting flaws in the weld zone; and a scanning step of switching the selected transducers one after another, the weld zone being subjected to ultrasonic testing by alternately repeating the flaw detection step and the scanning step, wherein in the scanning step, the selected transducers are switched one after another so that the range of an effective beam width of each of the switched selected transducers for a flaw to be detected has an overlapping portion, and wherein in the flaw detection step, the weld zone is subjected to flaw detection with a flaw detection sensitivity, which is adjusted beforehand for each of the selected transducers, so that maximum echo intensities from a flaw to be detected, which are each received by each of the selected transducers that have been switched, become substantially equal to each other, and wherein the effective beam width of each of the selected transducers for a flaw to be detected indicates the length of the range in which a flaw echo intensity becomes not less than a prescribed intensity in a profile of the echo intensity of a flaw to be detected which is obtained when each of the selected transducers is scanned in a direction orthogonal to the weld line of the weld zone. an arrangement step of arranging an ultrasonic probe (1, 1A, 1B) so as to face a weld zone of a test object, the ultrasonic probe (1, 1A, 1B) having n (n ≥ 2) transducers (11-1,..., 11-16) arranged along a direction orthogonal to a weld line of the weld zone of the test object; and a flaw detection step of selecting m (n > m ≥ 1) transducers from the n transducers (11-1,..., 11-16), and causing the selected transducers to transmit ultrasonic waves to the weld zone and to receive an echo from the weld zone, thereby detecting flaws in the weld zone; and a scanning step of switching the selected transducers one after another, the weld zone being subjected to ultrasonic testing by alternately repeating the flaw detection step and the scanning step, wherein in the scanning step, the selected transducers are switched one after another so that the range of an effective beam width of each of the switched selected transducers for a flaw to be detected has an overlapping portion, and wherein in the flaw detection step, the weld zone is subjected to flaw detection with a flaw detection sensitivity, which is adjusted beforehand for each of the selected transducers, so that maximum echo intensities from a flaw to be detected, which are each received by each of the selected transducers that have been switched, become substantially equal to each other, and wherein the effective beam width of each of the selected transducers for a flaw to be detected indicates the length of the range in which a flaw echo intensity becomes not less than a prescribed intensity in a profile of the echo intensity of a flaw to be detected which is obtained when each of the selected transducers is scanned in a direction orthogonal to the weld line of the weld zone.