Abstract: 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.