A two-terminal circuit having a relatively high series degree is configured by connecting a plurality of electricity storage elements to each other, and charging of the electricity storage elements is started by injecting a current into this two-terminal circuit. After the connection of the electricity storage elements is changed to reduce the series degree and a voltage variation among the electricity storage elements is solved at a predetermined timing, a connection change is made to configure a two-terminal circuit having a high series degree again and charging is continued. Each electricity storage element can be charged in a short time while voltages are balanced among the electricity storage elements.