An apparatus for evaluating the fastening state of a threaded joint of pipes or tubes, the threaded joint comprising a pin which has a shoulder part and in which an external thread part is formed, and a box which has a shoulder part buttable against the shoulder part of the pin and in which an internal thread part is formed, the external thread part of the pin being engaged with the internal thread part of the box, whereby the pin being fastened to the box, the apparatus comprising: a continuous length member which is adapted to be inserted from one opening end of the threaded joint into the threaded joint; a butting member which is mounted on the insert front side of the continuous length member to be butted against the opening end; a flat plate member which is mounted to the butting member to be supported on the internal surface of the threaded joint in a state where the butting member is butted against the opening end; a surface-wave transmission probe which is mounted on the insert rear side of the continuous length member turnably in the axial direction of the threaded joint, has a convex contact surface according to the internal surface geometry of either one of the pin and the box on which it is to be placed, and adapted to transmit ultrasonic surface waves; a surface-wave reception probe which is mounted on the insert rear side of the continuous length member turnably in the axial direction of the threaded joint, has a convex contact surface according to the internal surface geometry of the other one of the pin and the box on which it is to be placed, and adapted to receive ultrasonic surface waves; and an elastic member which is adapted to urge the surface-wave transmission probe and the surface-wave reception probe toward the internal surface of the pin or the box on which the respective probes are to be placed. a continuous length member which is adapted to be inserted from one opening end of the threaded joint into the threaded joint; a butting member which is mounted on the insert front side of the continuous length member to be butted against the opening end; a flat plate member which is mounted to the butting member to be supported on the internal surface of the threaded joint in a state where the butting member is butted against the opening end; a surface-wave transmission probe which is mounted on the insert rear side of the continuous length member turnably in the axial direction of the threaded joint, has a convex contact surface according to the internal surface geometry of either one of the pin and the box on which it is to be placed, and adapted to transmit ultrasonic surface waves; a surface-wave reception probe which is mounted on the insert rear side of the continuous length member turnably in the axial direction of the threaded joint, has a convex contact surface according to the internal surface geometry of the other one of the pin and the box on which it is to be placed, and adapted to receive ultrasonic surface waves; and an elastic member which is adapted to urge the surface-wave transmission probe and the surface-wave reception probe toward the internal surface of the pin or the box on which the respective probes are to be placed.