PURPOSE: To prevent the decrease in the yield of a product owing to the balging in the short sides of a continuously cast billet by retarding the movement of the short side support devices for the continuously cast billet provided below a mold from the movement of the short sides of the mold by a specific time in the stage of changing the widths on short sides of the billet in a continuous casting device.

CONSTITUTION: Short side support devices 3 for a continuously cast billet 5 consisting of plural rolls 4 are disposed below the bottom end 2 of short sides 1, 1 of a mold for continuous casting. The expansion of the width on the short sides of the mold 1 for continuous casting is accomplished by opening the devices 3 with driving devices 9 with the delay from the time for starting the movement of the bottom end 2 of the mold 1 by the delay time determined from the distance L2 between the bottom end 2 of the mold 1 and the top end of the devices 3 for the billet 5 and the casting speed V of molten metal. Conversely the reduction of the width on the short side is accomplished by narrowing the devices 3 in accordance with the delay time determined from the distance L1 between the bottom end 2 of the mold and the bottom end of the devices 3 and the speed V. The width on the short sides of the continuously cast billet is changed without bulging.