bottom has a plurality of vertical connecting holes 611 thereon the sidewalls have a plurality of horizontal connecting holes 612 thereon for connecting with the rods 41 42 below mentioned the connecting sleeve for connecting the lower beam 30 is opened upward while the connecting sleeve for connecting the upper beam 50 is opened downward in fig2 b the connecting block 62 which is a plate like body has a plurality of threaded holes 621 corresponding to the plurality of vertical connecting holes 611 on the connecting sleeve 61 as shown in fig3 the connecting block 62 is disposed in the lower beam 30 or the upper beam 50 at the position corresponding to the upright post 20 respectively where the connecting block in the lower beam 30 is mounted in the top portion inside of the lower beam while the connecting block in the upper beam 50 is mounted at the bottom of the upper beam and the connecting block is mounted into the upper beam or lower beam by a special fixture when the lower beam 30 and the upper beam 50 are connected with the upright post 20 one end of the connecting bolt 63 with a screw cap is disposed in the connecting sleeve 61 while the other end passes through the lower beam 30 or the lower beam 50 to threadedly connect with the connecting block 62 so that the lower beam 30 and the upper beam 50 could connect with the upright post as shown in fig2 a fig2 b and fig3 each door unit 10 is equipped with a pair of cross rod assemblies 40 on the front side and the rear side respectively to adjust the degree of tightness the rod assembly.