1. A defect inspecting apparatus for inspecting a defect on the outer peripheral surface of a pipe or tube in which in the end portion thereof, an external thread part and a lip part are provided in that order from the inside in the pipe or tube axis direction; a load face of the external thread part is inclined inwardly in the pipe or tube axis direction relative to the vertical plane perpendicular to the pipe or tube axis direction; and the lip part is tapered such that the dimension in the direction perpendicular to the pipe or tube axis direction decreases outwardly in the pipe or tube axis direction, the defect inspecting apparatus comprising: a first light source for illuminating the outer peripheral surface of the lip part, in which the optical axis thereof is inclined through an angle A satisfying formula (1) outwardly in the pipe or tube axis direction relative to the vertical plane; a first image capture device which is attached to the first light source so that the optical axis thereof is coaxial with the optical axis of the first light source, and receives the reflection light emitted from the first light source and reflected by the outer peripheral surface of the lip part to grab the image of the outer peripheral surface of the lip part; a second light source for illuminating the load face; a second image capture device which is attached to the second light source, and has the optical axis adjusted so that the reflection light emitted from the second light source and reflected by the load face to the direction inclined through an angle B satisfying formula (2) inwardly in the pipe or tube axis direction relative to the vertical plane can be received, and the reflection light is received to grab the image of the load face; a third light source for illuminating a thread bottom face inspection zone ranging from a boundary part between the load face and the thread bottom face of the external thread part to a portion of the thread bottom face spaced apart inwardly by a predetermined distance from the boundary part in the pipe or tube axis direction; a third image capture device which is attached to the third light source, and has the optical axis adjusted so that the reflection light emitted from the third light source and reflected by the thread bottom face inspection zone to the direction inclined through an angle C satisfying formula (3) inwardly in the pipe or tube axis direction relative to the vertical plane can be received, and the reflection light is received to grab the image of the thread bottom face inspection zone; and an inspection device for inspecting defects on the outer peripheral surface of the lip part, on the load face, and in the thread bottom face inspection zone by processing the captured images grabbed by the first to third image capture devices: 
 a− 45≦ A≦a+ 45  (1) 
 b<B≦c (2) 
 b<C≦d (3) a° being an angle (smaller than 90°) formed between the outer peripheral surface of the lip part and the pipe or tube axis direction in the cross section including the pipe or tube axis; b° being an angle (smaller than 90°) formed between the load face and the vertical plane in the cross section including the pipe or tube axis; c° being an angle (smaller than 90°) formed between a straight line and the vertical plane in the cross section including the pipe or tube axis, the straight line connecting the boundary part between the load face and the thread bottom face to a front end part of an insertion face of the external thread part, the insertion face connecting with an inside end part in the pipe or tube axis direction of the thread bottom face; and d° being an angle (smaller than 90°) formed between a straight line and the vertical plane in the cross section including the pipe or tube axis, the straight line connecting an inside end part in the pipe or tube axis direction of the thread bottom face inspection zone to the front end part of the insertion face of the external thread part, the insertion face connecting with an inside end part in the pipe or tube axis direction of the thread bottom face.