To solve problems on a joint joiner executed at a seam of each board such as a ceiling board or a wall board, wherein a method of mounting the joint joiner from the front and fixing it on the back is general, but it is difficult to install a scaffolding for execution from a lower part in the case of constructing a floor at sea, and a construction cost of the scaffolding is expensive, and in the case of a building with the high ceiling and walls, the construction cost of the scaffolding is expensive although the degree of difficulty is lowered.

A joint joiner device with a support bar turnably provided at a lower joiner body through a mounting component, is constituted so that the joint joiner can be installed being inserted from the ceiling or a floor board upper part after the installation of the ceiling board or floor board, or being inserted from the inside after the installation of the wall board.