To obtain a joint structure of a main reinforcement and a tie plate which enables easy joining between the main reinforcement and the tie plate and enables strong joining of the main reinforcement to the tie plate.

U-shaped notches 4 are formed in the outer edge of a horizontal tie plate 1. Recessed grooves 4c, 4c' are formed in surface parts 4a, 4a' facing each other. A holding metal fitting 5 is provided which comprises intermediate circular arc parts 5b, 5b' formed of a rotary shaft part 5a and both end parts thereof, and coming-off preventing leg parts 5c, 5c' formed by bending the tips of the intermediate circular arc parts 5b, 5b' in the directions opposite to each other. When the rotary shaft part 5a is engaged with one recessed groove 4c, the other recessed groove 4c' exists in a rotation path of the coming-off preventing leg parts 5c, 5c' around the rotary shaft part 5a as a center. In a state where the outer peripheral surface 6' of a main reinforcement 6 is fitted to a circular arc surface part 4b, the holding metal fitting 5 is rotated around the rotary shaft part 5a to engage the coming-off preventing leg parts 5c, 5c' with the inside of the other recessed groove 4c', and the outer peripheral surface 6' of the main reinforcement 6 fitted to the circular arc surface part 4b is held by the intermediate circular arc parts 5b, 5b' to fix the main reinforcement 6 to the tie plate 1.