Abstract: A secondary battery is charged with a large current after a time when a voltage across the secondary battery becomes equal to or greater than a decision value while the secondary battery is being charged, until a primary differential of the voltage reaches a decision value which is a threshold for the primary differential which is established depending on the temperature of the secondary battery. When the primary differential reaches the decision value at a time, the charging of the secondary battery with the large current is finished. By monitoring the quadratic differential, it is possible to distinguish an increase in the voltage in an initial charging period from an increase in the voltage in a terminal charging period. After an increase in the voltage in terminal charging period is detected, the end of the charging process is decided based on the decision value of the primary differential of the voltage.