Joining method and joint structure of members

A joint structure between a first member having a first hole portion and a second member having a hollow shape, the second member being inserted into the first hole portion, wherein

• the first hole portion has a polygonal cross-sectional shape,
• the second member has a cross-sectional shape corresponding to the polygonal cross-sectional shape of the first hole portion, the second member including a plurality of straight line portions extending linearly and a corner portion positioned between the adjacent two straight line portions in the cross-sectional shape, and
• an amount of enlarged deformation of the second member in at least the one corner portion is larger than an amount of enlarged deformation of the second member in the straight line on at least one side, wherein
• the straight line portion has a length that is equal to or more than a first length, and
• a core member is disposed at the straight line portion,
• the first length is equal to an effective width B of the first member, the effective width B is determined by formula (1) below, and
• a length C of the core member in a longitudinal direction of the straight portion is determined by formula (3) below:
• B=α×√E×t/√σy (1)
• 0.9≤α≤1.1 (2)
• C≥L−B (3)
• where α is specified by formula (2), E is a Young's modulus of the first member, t is a plate thickness of the first member, σy is a yield stress of the first member, and L is a length of the straight line portion.