An ultrasonic testing system for a steel pipe member having a curvature, comprising: a transmitting/receiving unit having a transmitting unit, which is adapted to transmit an ultrasonic wave to the welded surface of a welded portion and to the inside surface of the steel pipe member, said welded portion being in a longitudinal direction of the steel pipe member, wherein the ultrasonic wave is incident at the welded surface at an angle within the range from 33.2° to 56.8°, and a receiving unit, which is adapted to partly or entirely receive an echo reflected in a direction within the range from -12° to 16° to the mirror reflecting direction in the welded surface, wherein the transmitting unit and the receiving unit are composed of different groups of transducer elements on one, two, or more array probes disposed in a circumferential direction of the steel pipe member; and a controller for scanning the steel pipe member in a thickness direction by carrying out a control to change the groups of transducer elements corresponding to the transmitting unit and the receiving unit on the array probe or to change the angle of the array probe as well as controlling the angle of incidence of the ultrasonic wave to the steel pipe member in the respective transmitting and receiving waves so that the angles of incidence to the welded surface and the angle of the echo on the welded surface are kept within the ranges defined as to the transmitted wave and the received wave, respectively, whereby the controller is adapted to control an angle of incidence to the steel pipe member and a focus position by offsetting at least one of timing of wave transmission and timing of wave reception of the respective transducer elements in the group of transducer elements so that the angles of incidence to the welded surface and the angle of the echo on the welded surface are kept within the defined ranges, respectively. a transmitting/receiving unit having a transmitting unit, which is adapted to transmit an ultrasonic wave to the welded surface of a welded portion and to the inside surface of the steel pipe member, said welded portion being in a longitudinal direction of the steel pipe member, wherein the ultrasonic wave is incident at the welded surface at an angle within the range from 33.2° to 56.8°, and a receiving unit, which is adapted to partly or entirely receive an echo reflected in a direction within the range from -12° to 16° to the mirror reflecting direction in the welded surface, wherein the transmitting unit and the receiving unit are composed of different groups of transducer elements on one, two, or more array probes disposed in a circumferential direction of the steel pipe member; and a controller for scanning the steel pipe member in a thickness direction by carrying out a control to change the groups of transducer elements corresponding to the transmitting unit and the receiving unit on the array probe or to change the angle of the array probe as well as controlling the angle of incidence of the ultrasonic wave to the steel pipe member in the respective transmitting and receiving waves so that the angles of incidence to the welded surface and the angle of the echo on the welded surface are kept within the ranges defined as to the transmitted wave and the received wave, respectively, whereby the controller is adapted to control an angle of incidence to the steel pipe member and a focus position by offsetting at least one of timing of wave transmission and timing of wave reception of the respective transducer elements in the group of transducer elements so that the angles of incidence to the welded surface and the angle of the echo on the welded surface are kept within the defined ranges, respectively.