Method of manufacturing rectangular tube having stepped portion

A method of manufacturing a rectangular tube having a stepped portion comprising a series of sequential steps comprising:

• a first step of forming V-shaped grooves on the rectangular tube on a first pair of opposing surfaces and a second pair of opposing surfaces of an end thereof in a direction parallel to a longitudinal direction thereof;
• and
• a second step, after the first step, of successively pressing each of the surfaces having the V-shaped grooves formed thereon from outside to inside such that through contact of dies each having a flat planar surface corresponding to each of the first and second pair of opposing surfaces, the first pair of opposing surfaces are compressed, while the second pair of opposing surfaces are not being compressed, to thereby close the V-shaped grooves on the second pair of opposing surfaces, and thereafter the second pair of opposing surfaces are compressed, while the first pair of opposing surfaces are not being compressed, to thereby close the V-shaped grooves on the first pair of opposing surfaces,
• in a sequential manner, whereby the end of the rectangular tube is radially reduced as the stepped portion of a rectangular cross-sectional shape.