An ultrasonic testing apparatus for a pipe or tube end portion, comprising: an ultrasonic probe which is adapted to be disposed under the end portion of a pipe or tube laid in horizontal direction to face the pipe or tube end portion, the ultrasonic probe being adapted to transmit ultrasonic waves to the end portion of the pipe or tube and to receive the ultrasonic waves therefrom; and a probe holder housing the ultrasonic probe and adapted to be disposed under the end portion of the pipe or tube to face the pipe or tube end portion and to follow the pipe or tube which rotates in the circumferential direction when in operation, and a follow-up device to which the probe holder is attached, the follow-up device being configured to move the probe holder vertically and horizontally to maintain the vertical and horizontal positional relationship between the probe holder and the pipe or tube on the basis of a measurement result of displacement of the outer surface of the rotating pipe or tube, the probe holder comprising a coupling medium reservoir part (21, 22, 23) which surrounds a space between the ultrasonic probe and, when in operation, the end portion of the pipe or tube to contain a coupling medium therein, and the coupling medium reservoir part (21, 22, 23) comprising: a coupling medium reservoir part body into which the coupling medium is to be supplied; an annular bellows part which is attached to the upper side of the coupling medium reservoir part body so as to internally communicate with the coupling medium reservoir part body, and can expand and contract vertically; and an annular spacer which is attached to the upper side of the bellows part, and at least the upper surface of the annular spacer is a flat horizontal surface. an ultrasonic probe which is adapted to be disposed under the end portion of a pipe or tube laid in horizontal direction to face the pipe or tube end portion, the ultrasonic probe being adapted to transmit ultrasonic waves to the end portion of the pipe or tube and to receive the ultrasonic waves therefrom; and a probe holder housing the ultrasonic probe and adapted to be disposed under the end portion of the pipe or tube to face the pipe or tube end portion and to follow the pipe or tube which rotates in the circumferential direction when in operation, and a follow-up device to which the probe holder is attached, the follow-up device being configured to move the probe holder vertically and horizontally to maintain the vertical and horizontal positional relationship between the probe holder and the pipe or tube on the basis of a measurement result of displacement of the outer surface of the rotating pipe or tube, the probe holder comprising a coupling medium reservoir part (21, 22, 23) which surrounds a space between the ultrasonic probe and, when in operation, the end portion of the pipe or tube to contain a coupling medium therein, and the coupling medium reservoir part (21, 22, 23) comprising: a coupling medium reservoir part body into which the coupling medium is to be supplied; an annular bellows part which is attached to the upper side of the coupling medium reservoir part body so as to internally communicate with the coupling medium reservoir part body, and can expand and contract vertically; and an annular spacer which is attached to the upper side of the bellows part, and at least the upper surface of the annular spacer is a flat horizontal surface. a coupling medium reservoir part body into which the coupling medium is to be supplied; an annular bellows part which is attached to the upper side of the coupling medium reservoir part body so as to internally communicate with the coupling medium reservoir part body, and can expand and contract vertically; and an annular spacer which is attached to the upper side of the bellows part, and at least the upper surface of the annular spacer is a flat horizontal surface.