To efficiently manufacture a steel pillar having a thickness part excellent in dimensional accuracy.

By heating the necessary position of a square pipe 2 to the vicinity of the lower limit temp. of a temp. at which crystal grains are coarsened, also heating a short square sleeve 8 which is possible to fit on the outside of the square pipe 2 to a working temp. at which the sleeve is plastically deformed, fitting the sleeve 8 on the heated part H of the square pipe 2 and pressing so that the sleeve 8 is bitten into the heated part H of the square pipe 2 from the outside using press dies 9, 10, a thick thickness part consisting of the heated part H and the sleeve 8 is formed. Successively, by cooling the heated part H of the square pipe 2 in the thick thickness part from the inside so as to get an intermediate temp. between the lower limit temp. of the temp. at which the crystal grains are coarsened and the ordinary temp., also rapidly cooling the sleeve 8 to the vicinity of the lower limit temp. of the temp. at which the crystal grains are coarsened and, after that, executing slow cooling, the sleeve 8 is shrinkage fit on the square pipe 2 by temp. difference between the heated part H and the sleeve 8.