Abstract: Disclosed is a core for assembling a cross member of a battery case. A core for assembling a cross member of a battery case is insertable into a closed section in a longitudinal direction of the cross member to laser-weld at least one part to be assembled to the cross member of the battery case with the closed section. The core may include: a first core block made of steel, which includes a first slip surface formed at one side in a width direction to be inclined in a direction toward the other side; a second core block made of the steel, which includes a second slip surface formed to be inclined at the one side in the width direction to slip on the corresponding first slip surface in an upper and lower direction; a plurality of first magnet members installed on one surface of the first core block; and a plurality of second magnet members installed on one surface of the second core block facing the one surface of the first core block.