To easily and securely perform connection work of a plurality of frame bars at a work site etc., to prevent connection portions of the frame bars from obstructing the work etc. and to improve appearance of the connection structure of the frame bars.

Fastening members are provided between the plurality of frame bars and a connection member, and a pair of locking parts are provided in each of the plurality of frame bars. A pair of engagement parts engaged with the pair of locking parts of the frame bar are provided in each of the fastening members. In the connection member, a plurality of fitting recessed parts and storage recessed parts are provided to be continued to each other and correspond to the plurality of frame bars and the plurality of fastening members. A plurality of locking bolts are inserted into the connection member from the sides opposite to the respective frame bars toward the respective fastening members, and are screwed between the pairs of engagement parts of the respective fastening members. By fastening the respective locking bolts, the respective fastening members are drawn to the back parts of the storage recessed parts to make the plurality of frame bars come into pressure contact with the respective fitting recessed parts.