A waste lithium-ion battery processing system is a system for processing a waste lithium-ion battery, the system performing roasting a waste lithium-ion battery whose cathode active material contains phosphorus to obtain a roasted product and immersing the roasted product in water to elute lithium from the roasted product into the water, thereby recovering the lithium from the waste lithium-ion battery. The system includes: mixing equipment that mixes a non-lithium alkali metal salt into the cathode active material contained in the waste lithium-ion battery; and a roaster that roasts, at a predetermined first temperature, a mixture that is obtained from mixing by the mixing equipment. The mixing equipment kneads the alkali metal salt into the cathode active material while grinding the alkali metal salt.