PURPOSE: To decrease the rolling load to be acted on shafts for fixing a pair of casting drums in a device for casting continuously a thin metallic sheet by the rotation of said casting drums by disposing a backup roll on the outside of at least one of the casting drums.

CONSTITUTION: A thin metallic sheet is continuously cast by the rotation of a pair of casting drums, i.e., a casting drum 1 having a large diameter and a casting drum 2 having a small diameter. A backup roll 3 is disposed on the outside of the drum 2. Then the relief of the drum 2 in the stage of rolling the slab 4 is prevented. The spacing between the casting drums facing to each other is thus maintained constant and the thickness of the slab is made uniform.