To provide a corrugated cardboard box of high precision capable of accurately folding a main rule.

The rule structure is formed to corrugated cardboard Q under pressure so as to be provided along the stepped parts of the core Q1 of the corrugated cardboard Q. Adjacent recessed parts depressed into an almost corrugated cross section are alternately formed on both left and right sides of the main rule along the stepped parts under pressure. Fitting parts are provided at the parts left without being pressed. When the corrugated cardboard is folded along the main rule, the fitting parts are provided so as to be fitted in the opposed adjacent recessed parts. The adjacent recessed parts are extended in the direction right-angled to the main rule. The width of the bottom parts of the adjacent recessed parts is formed so as to be made wider than that of the top parts of the fitting parts.