PURPOSE: To prevent a bend part from being flattened, and also to reduce a wall thickness of a tube by executing the bending in a state that a variation of width of a panel tube has been detained.

CONSTITUTION: Plural pieces of panel tubes 1 are set to a push tool 2 by detaining a mandrel 6 from the direction of a fin part 5 of the panel tube 1 through a spacer 9, a detaining jig 10 and fittings 11, etc. This is one example of a method for giving a detaining force from the direction of the fin part 5 of the panel tube 1, and other method is also available. In this way, by pressing the push tool 2 to which the tube 1 has been set, against receiving tools 3, 3, bending is executed. In this way, a bend part can be prevented from being flattened.