An aluminum alloy sheet is manufactured by preparing a slab having a thickness of 5 to 15 mm with a continuous casting machine by a continuous casting process using molten alloy containing 0.40% to 0.65% of Mg, 0.50% to 0.75% of Si, 0.05% to 0.20% of Cr, and 0.10% to 0.40% of Fe, a remainder being Al; winding the slab into a coil; cold-rolling the slab into a sheet; subjecting the sheet to solution heat treatment in such a manner that the sheet is heated to a temperature of 530° C. to 560° C. at a heating rate of 10° C./sec or more and then maintained at the temperature for five seconds or more; quenching the sheet with water; coiling up the sheet; maintaining the sheet at a temperature of 60° C. to 110° C. for 3 to 12 hours; and then cooling the sheet to room temperature.