In a box assembly device, a holding mechanism causes one of bottom boards to contact a first jig and folds the bottom board inside, while the holding mechanism holding the box material in a state where the box material is developed in a cylindrical shape from a folded state. The folding mechanism folds the bottom flap inside so as to overlap with one of the bottom boards. The holding mechanism causes the other bottom board to contact the second jig while holding the box material and folds the other bottom board inside and upwardly so as to overlap with the bottom flap to engage the bottom boards with each other.