Readily joining CLT panels to obtain a box-shaped building unit.
An orthogonal part of each surface of a rectangular parallelepiped has a structure wherein a surface of edge of lumber 11s of a CLT panel 11 is made abut on an inner main surface 10i of a CLT panel 10 and joined thereto by a joining member 2 so that a surface of edge of lumber 10s of the CLT panel 10 and an outer main surface 11o of the CLT panel 11 are flush. A joining part by the joining member 2 has a structure wherein the surface of edge of lumber 10s and the inner main surface 10i and outer main surface 10o of the CLT panel 10 are opened so that the outer main surface 10o side is wider than the inner main surface 10i side, the surface of edge of lumber 11s and at least the outer main surface 11o of the CLT panel 11 are opened so that the inner side of the CLT panel 11 is wider than the surface of edge of lumber 11s side, and the butterfly joint-shaped joining member 2 is inserted into the opening of the CLT panel 10 and the opening of the CLT panel 11 from the surface of edge of lumber 10s of the CLT panel 10 and the outer main surface 11o of the CLT panel 11 side.