To provide a packaging unit made of cardboard or corrugated fiberboard, its base cardboard and method of assembling the packaging unit, which enable the self-standing retention of difficult-to-stand-alone containers taken out, causing few useless portions to occur at the time of cutting, and which can be handled on an automated assembly line known to the public and in practice from the past.

The base cardboard for the packaging unit made of cardboard or corrugated fiberboard, which consists of a bottom board (a), front, rear, right and left lateral boards (b), (c), (d), (h), provided in communication therewith, holding or protective secondary boards (e), (f) provided in communication with the left lateral board (d), holding or protective secondary boards (i), (j) installed in communication with the right lateral board (h), a holding or protective main board (g) removably connected to the left lateral board (d) by means of a connecting portion 31, a lid board (k) provided in communication with the right lateral board (h), and front and rear lateral boards (m), (n) and a fixed strip (p) provided in communication with the lid board (k).