[Problem] To provide a means for improving a wall surface structure of a vacuum molding mold to facilitate demolding of a plastic molded article having an undercut. [Solution] Provided is a vacuum molding mold for vacuum-molding a deep-dish-shaped tray having an inner partition wall and an undercut in a lateral wall inner surface and/or a lateral surface of the partition wall, the undercut providing a support base for stacking the trays in multiple stages. The vacuum molding mold comprises an undercut forming part for forming the undercut which is provided with: a movable block protruding from the lateral wall inner surface and/or the lateral surface of the partition wall; a housing or a cavity part accommodating the movable block on the inside of the lateral wall inner surface and/or the lateral surface of the partition wall; a spring held between a back surface of the movable block and a back surface of the housing or the cavity part; and a support shaft rotatably supporting the movable block. An upper part of the movable block is pressed into the mold during demolding.