To stables cast a thin sheet for a long time in a twin drum type continuos thin sheet casting method using a pair of cooling drum and a side gate.

This brazing filler material 10 whose viscosity becomes 5 to 100 poise in a melting state of 150°C or over, containing lubricant by 50 to 95%, is applied by 50 to 300 nm in advance on a sliding surface 18 of the side gate of a cooling drum 1a. It is preferable that the lubricant is BN, graphite, mica, barium chromate or boric acid except inevitable impurities.