To provide a joint structure of a steel pipe column excellent in executability, and reducing a cost.

Studs 5a are fixed in parallel by welding to respective four surfaces of a lower end part of an upper square steel pipe 2. Similarly, studs 5b are fixed by welding to four surfaces of an upper end part of a lower square steel pipe 3. Holes 7 are arranged at the same arranging interval as the studs 5a and 5b in a joint plate 6 for connecting a joint part 4 of the upper square steel pipe 2 and the lower square steel pipe 3. The lower square steel pipe 3 is built up at a job site, and is placed on the upper end so that the lower end part of the upper square steel pipe 2 is coaxially opposed to the lower square steel pipe 3, and then, the joint plate 6 is installed and temporarily fixed so as to penetrate through both the studs 5a installed in the lower end of the upper square steel pipe 2 and the studs 5b installed in the upper end of the lower square steel pipe 3. After applying similar work to the respective four surfaces of the joint part 4, the joint plate 6 is fixed to the lower square steel pipe 3 and the upper square steel pipe 2 by using nuts 8, and a square steel pipe column 1 is executed.