A posture control method controls a posture of a processing unit by measuring a distance between the processing unit and a process surface of a round pipe to be processed by the processing unit, using four distance measuring units. The method controls a posture about a pitch axis based on 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 about a yaw axis based on 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 about a roll axis based on 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.