To efficiently produce a steel frame column having a thickness part excellent in dimensional precision.

An end of a square tube 2 is heated and a thick wall female side fitting part 9 having an inclining face 10c whose outside face is made parallel with the square tube 2 and whose inside face is broadened toward the top end is formed with pressing by using a female side outside die and a female side inside die on the heated end, an end of another square tube 2a is heated and a male side fitting part 15 having an inclining face 14c being made slender toward the top end and matched to the inclining face 10c of the female side fitting part 9 is formed with pressing by using a male side outside die and a male side inside die on the heated end, the male side fitting part 15 is entered forcibly in the inside of the female side fitting part 9 and the thickness part 7 is formed, and the top end part of the female side fitting part 9 is fixed with welding 27 onto the male side fitting part 15.