To prevent lateral dislocation after connection by arranging a swelling part and a recessed part having inclined faces in a connecting part of a horizontal member, arranging a projecting part and a recessed part in the axial direction of a connecting surface, and connecting them to each other by fitting the projecting part and the recessed part of both members to each other.

A swelling part 6 having an inclined face 3a and a recessed part 7 having an inclined face 3b are arranged on one connecting surface 1a of a horizontal member, and a continuous projecting part 8 is arranged over a tip surface part 2 and a base end surface part 5. The swelling part 6, the recessed part 7 and a recessed part 9 press-fitted to the connecting surface 1a are arranged on the other connecting surface 1b of the horizontal member. The connecting surfaces 1a and 1b of both members are integrally formed by press-fitting and fitting them together, and a tenon is formed of the projecting part 8 and the recessed part 9, and lateral dislocation is made uncausal to force from the side surface direction of the horizontal member. The connecting surfaces 1a and 1b are worked by a series of operation of a rotary cutter in a factory or the like. Therefore, a connecting part resistant to lateral dislocation can be easily obtained.