Abstract: To provide a method of producing a strengthened alloy capable of shortening the time necessary for aging treatment and obtaining a strengthened alloy with enhanced tensile strength. The method of producing a strengthened alloy comprises a solution treatment step S1 of immersing an alloy material into molten lithium held at a solution treatment temperature higher than solution temperature of solute metal of the alloy material, a solution stop step S2 of immersing the alloy material into molten lithium held at a cooling temperature lower than the solution treatment temperature after the solution treatment step S1, an aging treatment step S3 of immersing the alloy material into molten lithium held at an aging treatment temperature lower than the solution temperature after the solution stop step S2, and an aging stop step S4 of immersing the alloy material into molten lithium held at an aging stop temperature lower than the aging treatment temperature after the aging treatment step S3.