This invention discloses a width-variable mold device to be used by placing a feed device for moving minor side members toward a movable direction and by varying steplessly the width of the mold within a certain range by means of the operation of the feed device, in a mold device formed in an approximately rectangular shape by both two mutually opposed major side members having slightly curved opposed surfaces so that their opposed interval may reach its maximum at the center portion and its minimum at both ends and two opposed minor side members bringing both side ends into contact with the opposed surfaces of the major side members and placing between the major side members, wherein the major side members and the minor side members are equipped with their water cooled jackets at their outside, each of the major side members is suspended and supported to the support tables at both sides so as to move each of the major side members toward the direction where their opposed interval varies, and at the same time, each of the minor side members is suspended and supported to the upper portions of the major side members so as to move each of the minor side members toward the direction where their opposed interval varies.