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 cardboard box sheet (a sheet) in which first and second side surfaces (C11, C12) and third and fourth side surfaces (C13, C14) overlap, and forms a square-tube-shaped cardboard box. The box-opening device has a suction disk (41b) to grasp the first side surface, a main arm (45a) having suction disks (42a, 42b) 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. The main arm moves at or slower than a predetermined speed in the initial action, and moves faster than the predetermined speed in the main action. The control part controls the main arm based on an angle ( ± ) formed by the first side surface and the fourth side surface at completion of the initial action, which is determined according to the sheet size.