Provided is a method by which the degraded performance of a lithium ion secondary battery containing phosphorus atoms (P) in a nonaqueous electrolytic solution can be restored by subjecting the lithium ion secondary battery in which a coating film including P is formed on a positive electrode surface, to a comparatively simple treatment. The degraded performance recovery method for a lithium ion secondary battery disclosed herein includes an ultrasound treatment step of applying ultrasound to the lithium ion secondary battery. In the ultrasound treatment step, the frequency of the generated ultrasound is 900 kHz or higher, and the period of time in which the ultrasound is applied to the lithium ion secondary battery continuously is 5 min or more.