A box forming apparatus includes first holders configured to hold a first side surface of a cardboard sheet, second holders configured to hold a second side surface of the cardboard sheet, an arm, a first mover, and a second mover. The arm has a support that supports the first and the second holders. The first mover changes a position of the first holders relative to the second holders while the arm is being rotated to open the cardboard sheet. The second mover moves the arm in association with opening of the cardboard sheet so that, when the cardboard sheet has been opened, the second mover will have moved the support a predetermined distance in a second direction opposite a first direction, which is a direction heading from a rotational center of the arm to a distal end of the arm.