A plurality of short light metal billets obtained by pressing light metal pieces are stacked in a long container having an inside diameter D that is larger than an outside diameter d of each of the short light metal billets, are pressed in the long container at a temperature higher than room temperature, and are compressed until the outside diameter d of each of the short light metal billets becomes equal to the inside diameter D of the long container, thereby joining the short light metal billets together at an interface between each adjacent pair of the short light metal billets by friction.