To provide a joint structure object for improving strength in rod-shaped members.

The insertion part 21a of an engaged member 21 is inserted to one of a plurality of engagement grooves 12 of the rod-shaped member 10, which are formed inward from an external periphery along a longitudinal direction. The rod-shaped member 10 is inserted to a fixing member 30, while allowing projection parts 21b projected from both sides of the insertion part 21a to approach inward. Thus, the engaged member 21 is joined, while fixing the rod-shaped member 10 to the fixing member 30. Since a screw hole is not bored in the end surface of the rod-shaped member 10, the engagement grooves 12 are deeply formed for the portion. When it is assumed that a uniform load (a surface load) is added to the engaged member 21 supported by the two rod-shaped members 10, orthogonal stress to act on the wall surface of the engagement groove 12 is reduced in inverse proportion to the depth of the engagement groove 12. That is, the strength of the rod-shaped member 10 is improved by allowing the screw hole not to be bored in the end surface of the rod-shaped member 10.