Provided is a highly reliable box-opening device being capable of suppressing failures of a box-opening action regardless of a size of a cardboard box sheet. The box-opening device unfolds a sheet in which first and second side surfaces and third and fourth side surfaces overlap, and forms a square-tube-shaped cardboard box. The box-opening device may have a suction disk to grasp the first side surface, a main arm having suction disks to grasp the fourth side surface and changing a posture while grasping the fourth side surface, and a control part to control a movement of the main arm so that an initial action, in which a gap is formed between the first and second side surfaces and the third and fourth side surfaces, and a main action that follows the initial action, are executed as an action of unfolding the sheet.