PURPOSE: To permit a solid box to be easily formed of a flatly folded sheet material by making up wall and bottom parts of a box of a polypropylene material and bonding together at least the bonding parts of the bottoms and the bonding pieces of the walls using cyano adhesive agent.

CONSTITUTION: Nearly trapezoidal notches 14 and 24 are provided with interengageable slits 15 and 25, respectively. The bottom pieces 12 and 22 having folding lines C extending from these notches to the lower ends of folding lines A and bonding parts 13 and 23 are provided extending from the lower edges of a front wall part 1 and a back wall part 2. Bonding pieces 8 and 9 are provided extending from the lower edges of side wall parts 3 and 4. At least the bonding parts 13 and 23 of the bottom pieces 12 and 22 and the bonding pieces 9 and 8 of the side wall parts 4 and 3 are bonded together using cyano adhesive agent. When the folded box thus formed are unfolded into a solid form, the notches 14 and 24 of the bottom parts 12 and 22 are engaged with each other and the slits 15 and 25 thereof are also engaged with each other to permit a solid box to be formed with one touch operation.