A speed synchronization system in an aluminum alloy slab continuous casting and rolling line having a twin-belt casting machine and skin pass rolling machine which suitably controls a belt speed of the twin-belt casting machine and roll speed of the skin pass rolling machine so as to obtain a sound aluminum alloy continuously cast and rolled slab and a production facility and method of production of an aluminum alloy continuously cast and rolled slab using the same. These (1) compare an actually measured value of a slab speed during advance from a twin-belt casting machine to a skin pass rolling machine with a line speed setting to control a roll speed of a skin pass rolling machine by proportional/integrated control and simultaneously (2) control a belt speed of the twin-belt casting machine based on the line speed setting and a solidification shrinkage rate of the aluminum alloy to be cast so as to synchronize the belt speed of the twin-belt casting machine and the roll speed of the skin pass rolling machine.