A magnesium alloy coil stock produced by coiling a sheet composed of a magnesium alloy in a cylindrical shape, wherein: the internal diameter of the coil stock is 1000 mm or less, the magnesium alloy contains Al as an additive element in an amount of 8.3% or more by mass and 9.5% or less by mass, and the coil stock satisfies the amount of warpage in a width direction below: when the amount of warpage in a width direction (%) is defined by (maximum distance h in vertical direction/width w of test piece for warpage amount) × 100, the amount of warpage in a width direction is 0.5% or less, wherein, when a sheet located on an outermost peripheral side of the sheet constituting the coil stock is cut into a length of 300 mm to obtain a test piece for warpage amount and the test piece for warpage amount is placed on a horizontal table, the maximum distance in a vertical direction between a surface of the horizontal table and a portion of one surface of the test piece for warpage amount, the portion being not in contact with the horizontal table, in a width direction of the test piece for warpage amount is referred to as h and the width of the test piece for warpage amount is referred to as w, the magnesium alloy coil stock obtainable by a method comprising: a preparation step of preparing a raw material coil stock obtained by coiling a raw material sheet composed of a magnesium alloy in a cylindrical shape; a warm working step of continuously feeding the raw material sheet by uncoiling the raw material coil stock and working the fed raw material sheet while the raw material sheet has a temperature of more than 100°C; and a coiling step of coiling the worked sheet to form a coil stock whose internal diameter is 1000 mm or less, wherein the coiling step is performed after the temperature of the worked sheet just before coiling is decreased to 100°C or less, and wherein the magnesium alloy contains Al as an additive element in an amount of 8.3% or more by mass and 9.5% or less by mass. the internal diameter of the coil stock is 1000 mm or less, the magnesium alloy contains Al as an additive element in an amount of 8.3% or more by mass and 9.5% or less by mass, and the coil stock satisfies the amount of warpage in a width direction below: when the amount of warpage in a width direction (%) is defined by (maximum distance h in vertical direction/width w of test piece for warpage amount) × 100, the amount of warpage in a width direction is 0.5% or less, wherein, when a sheet located on an outermost peripheral side of the sheet constituting the coil stock is cut into a length of 300 mm to obtain a test piece for warpage amount and the test piece for warpage amount is placed on a horizontal table, the maximum distance in a vertical direction between a surface of the horizontal table and a portion of one surface of the test piece for warpage amount, the portion being not in contact with the horizontal table, in a width direction of the test piece for warpage amount is referred to as h and the width of the test piece for warpage amount is referred to as w, the magnesium alloy coil stock obtainable by a method comprising: a preparation step of preparing a raw material coil stock obtained by coiling a raw material sheet composed of a magnesium alloy in a cylindrical shape; a warm working step of continuously feeding the raw material sheet by uncoiling the raw material coil stock and working the fed raw material sheet while the raw material sheet has a temperature of more than 100°C; and a coiling step of coiling the worked sheet to form a coil stock whose internal diameter is 1000 mm or less, wherein the coiling step is performed after the temperature of the worked sheet just before coiling is decreased to 100°C or less, and wherein the magnesium alloy contains Al as an additive element in an amount of 8.3% or more by mass and 9.5% or less by mass. a preparation step of preparing a raw material coil stock obtained by coiling a raw material sheet composed of a magnesium alloy in a cylindrical shape; a warm working step of continuously feeding the raw material sheet by uncoiling the raw material coil stock and working the fed raw material sheet while the raw material sheet has a temperature of more than 100°C; and a coiling step of coiling the worked sheet to form a coil stock whose internal diameter is 1000 mm or less, wherein the coiling step is performed after the temperature of the worked sheet just before coiling is decreased to 100°C or less, and wherein the magnesium alloy contains Al as an additive element in an amount of 8.3% or more by mass and 9.5% or less by mass.