A posture control method uses four distance measuring means to control a posture of processing means by measuring a distance between processing means and a processing surface of a round pipe to be processed by processing means. The method controls a posture around a pitch axis according to a difference obtained by subtracting a second measurement distance from a first measurement distance and a difference obtained by subtracting a fourth measurement distance from a third measurement distance; controls a posture around yaw axis according to a difference obtained by subtracting the third measurement distance from the first measurement distance and a difference obtained by subtracting the fourth measurement distance from the second measurement distance; and controls a posture around a roll axis according to a difference between a first total distance obtained by adding the second measurement distance and the third measurement distance and a second total distance obtained by adding the first measurement distance and the fourth measurement distance.