To surely fit a bolt from the inner side of a steel tube column into a bolt hole cut in a side surface of the steel tube column with a simple operation without providing a large insertion hole which may damage the strength of the steel tube column in the side surface of the steel tube column.

A tool 1 for fitting a bolt to fit a bolt 6 from the inner side of a steel tube column 5 into a bolt hole 51 cut in a side surface of the steel tube column 5, is provided with a screw part 2 to be screwed in a tip part of the bolt 6 in an attachable/detachable manner, a long insertion guide part 3, and a connection part to connect the screw part 2 to a rear end of the insertion guide part 3, and the connection part 4 comprises a wire and a synthetic resin tube to cover the wire. The insertion guide part 3 is inserted in an operation insertion hole 52 provided in the side surface of the steel tube column 5 from its tip 3a so that the tip 3a is guided in the specified bolt hole 51 by the operation from outside the steel tube column 5.