To provide a box-making machine which folds a side flap by rotatably operating a guide plate as a part of a portion for sharing the support of a blank in a waiting state and which has a simple structure and an easy box- making work.

The box-making machine comprises a first roller 26 and a second roller 28 raising from below in a state in which the rollers are placed on an elevation base 22, rotating and standing a first folding guide 11, and a second folding guide 13 toward an inside while the rollers 26 and 28 are respectively rolled and engaged with the first guide 11 and the second guide 13. The side flaps b1, b2 of the blank supported in a flat plate state without inner die are smoothly folded up to manufacture the case. Corner flaps b3, b4 folded out are superposed on the side flap b2 by a folding mechanism 40 having folding rollers 42.