A packaging machine includes a bottom breaker which has first to third prefolding members (41) to (43). The third prefolding member (43), serving to prefold a fold forming portion (27) by bending this portion (27) outward along a base part thereof, is pivotally movable from a nonoperative position P0 to a second operative position P2 via a first operative position P1. The fold forming portion (27) is moved with a first bottom panel (21) by the operation of the second prefolding member (42) so as to be brought into contact with the third prefolding member (43) and thereby bent outward while the third prefolding member (43) is pivotally moved from the nonoperative position P0 toward the first operative position P1. The third prefolding member (43) is pivotally moved from the first operative position P1 toward the second operative position P2 after the operation of the second prefolding member (42) to thereby bend the fold forming portion (27) through an increased angle.