A charger for a secondary battery including a positive electrode, a negative electrode including lithium-containing silicon represented by the composition formula Li.sub.xSi, and an electrolyte. This charger includes: (1) a voltage detector that detects voltage of the secondary battery that is being charged; and (2) a charge controller that calculates the value x in Li.sub.xSi included in the secondary battery from an output of the voltage detector and stops the charging of the secondary battery when the calculated value x reaches a predetermined threshold value. The charge controller has at least one settable threshold value including a first threshold value, and the first threshold value is 2.33 or less. The use of this charger makes it possible to charge the secondary battery such that it has a high capacity, or a higher capacity if necessary, without accelerating the deterioration of cycle life.