PURPOSE: To maintain a standing state of a side wall without warping a paper box in a paper box for packing wherein the side walls extending from around a bottom wall and made to stand at the time of packing can be restored to a state before standing at the time of unpacking.

CONSTITUTION: Cuts 24 are provided from right and left side rims of fold pieces 21 formed outside opposite side walls, cuts 34 are provided from an outer rim of flaps 31 protruding on both sides of another pair of opposite side walls 2, 4, and when the flaps 31 are inserted between the fold pieces 21 and the side walls 3, 5, the fold pieces 21 and the flaps 31 bring cut rims 25, 35 of the cut 24 and the cut 34 crossingly into contact with each other via the cuts 24, 34. Thus an object to be packed can be easily received while the side walls are standing, and also the side walls can be kept locked with each other, thereby advantageously obtaining a stable packed state.