In the metal billet to be used for hot dilation forming, a forward side with respect to the pressing direction has a quadrate section and its diagonal length is not more than an inner diameter of a container. Moreover, a backward side with respect to the pressing direction has a circular section and its diameter is substantially same as the inner diameter of the container. The metal billet is heated to a temperature suitable for press working and is set into a container for press forming. While a center of a workpiece of the metal billet is being bored by a boring punch to be operated by a pressing machine, the metal billet is hot-dilated so that a bottomed container for a cask is formed.