Provided are a lithium-ion secondary battery capable of measuring the concentration of lithium ions in an stored electrolyte in a predetermined portion, an assembled battery using the same, a vehicle and a battery-equipped device equipped with the battery or the assembled battery, a battery system capable of acquiring the concentration-correlated physical quantity in the lithium-ion secondary battery, and a method for detecting the deterioration of the lithium-ion secondary battery. A lithium-ion secondary battery comprises a power generation element including a positive electrode plate and a negative electrode plate, a battery case housing the power generation element, and an electrolyte containing lithium ions and held in the battery case, and further comprises a stored-electrolyte physical quantity measuring means capable of measuring the concentration-correlated physical quantity having a correlation to the concentration of lithium ions in the stored electrolyte stored between the element and the case.