To improve the throughput capacity of a box making machine which folds one after another a plurality of bottom flaps of a box material opened in a rectangular tube shape for making a box body.

A folding member 106 for an inside flap F1 to be folded earlier is made of a thin panel shape member 106a extending in a face between and parallel to the facing edges of outer flaps F2, F4 folded later. The folding member 106 point contacts or line contacts the flap F1 at its center in the width direction when the flap F1 is completely folded, and does not bother the folding action of the outer flaps F2, F4 folded from both sides. As a result, when the folding of the inside flap F1 is completed, soon the folding of the outer flaps F2, F4 can be started. This way a box body Y is made without delay and results in the improvement of the throughput capacity of the box making machine.