To reduce the occurrence degree of shrinkage hole and shrinkage pipe that the center part is extremely recessed at the solidified time by cooling molten alkali metal in a forming vessel while regulating a Y value having a specified relation in specified range.

In the case of using H (mm) for height of the cast alkali metal, (d) (mm) for inner diameter of the forming vessel at H/2 height, (h) (mm) for effective height of a cooling jacket, the molten alkali metal is cooled under condition, in which Y value in the equation Y=(H-h).d/h (mm) becomes 3-25, desirably 8-15. Coolant is circulated in the cooling jacket 7 and the inner part in the forming vessel body 2 is replaced with argon gas and the molten alkali metal having a prescribed temp. is poured by a prescribed quantity. In the case of lithium, molten lithium is held at e.g. 200-270°C and in the case the total casting quantity is e.g. 0.5-3.0 kg, the casting speed is desirable to be 10-30 g/sec. The forming vessel desirably has a truncated conical shape and as the desirable coolant oil is used at 15-40°C.