A charger 10 includes an output circuit unit 3 to output a charging current to a secondary battery 21, a voltage detection unit 2 to detect a voltage of the secondary battery, and a control unit 1 to control the output circuit unit, whereby constant current charging and constant voltage charging are performed. The control unit decreases a constant current value of the charging current by stages during the constant current charging and determines that the secondary battery has deteriorated by using a first voltage drop value of the secondary battery occurring upon first conversion and a second voltage drop value of the secondary battery occurring upon second conversion.
