PURPOSE: To prevent ununiform thickness of a tube and bad surface property from generation by detecting an amount of displacement at the tip of a mandrel, deciding whether it does not reach a tolerance and always controlling the direction of the tip of the mandrel bar to a fixed direction.

CONSTITUTION: The apparent amount of displacement from a shaft center is detected by a detector 1 for the amount of displacement at the tip part of the mandrel bar. A signal for the amount of displacement from the detector 1 for the amount of displacement at the tip part of the mandrel bar is cpmpared by a means 5 for deciding conformity of the amount of displacement at the tip part of the mandrel bar with a peset allowable amount of displacement to decide the conformity of the amount of displacement. When the amount of displacement does not exceed the allowable value, the maximum displacement direction at the tip part of the mandrel bar is controlled by a means 6 for controlling the maximum displacement direction at the tip part of the mandrel bar so that it is fixed by signals from the decision means 5 and a signal in this case. In this way, the bent state at the tip part of the mandrel bar can be grasped and the bent direction can be controlled to be fixed.