To solve problems of a structure in which a bottom lid is locked with lock pieces, that a bottom lid of a cardboard box is easily loosen due to breaking or bending of the lid because the lock relies on the engagement of small protruding pieces such as the lock pieces, and when the bottom of the cardboard box is closed by insertion of the lock pieces, an evenness is generated in the bottom surface, and when a relatively heavy product is accommodated in the box, a surface of the product is stamped or scratched by the lock pieces.

A cardboard box including a circumference surface formed with opposing length-side surfaces and opposing width-side surfaces, and a flap extending from one end of the circumference surface, that forms the bottom lid. The flap is integrally constructed so that it is sectioned by folding scores into adjacent sections. A reverse folding score is provided at the bottom lid forming flap on the width-side surface at 45 degrees from both ends of border of the width-side surface. Further, an incision is formed in the width-side surface so that it is originated from at least one end of the reverse folding score, and reaches a free end of the width-side surface. Further, a push-in piece is formed, which is provided with the bottom lid forming flap on the width surface side, and has an exposed portion formed between the reverse folding score and the incision, onto which the bottom lid forming flaps are not overlapped.