A structural member joint structure can easily join members to each other by using a simple structure having a reduced number of parts and can provide the members with a large fastening force which makes the members less likely to be loosened from each other. The joint structure is used to join one pillar and the other pillar to each other such that the other member is vertically abutted against a side face of the one pillar. The structure has a pair of fastening members positioned in the inside of the other pillar and arranged so as to be movable in the widthwise direction of a groove formed in a side face of the one pillar, a nut mechanism located on the inward side of the other pillar and formed between the pair of fastening members, and a bolt operated from the outside through an opening in the other pillar to be fitted into the nut mechanism. Rotating the bolt causes the nut mechanism to urge and move the pair of fastening members in the direction in which the pair is separated from each other. This causes engagement projections respectively arranged on ends of the pair of fastening members to engage with engagement recesses respectively arranged at lateral opposite ends of the groove in the one pillar. Thus, the pillars are joined to each other.