A state of charge estimation method includes: measuring a voltage of a lithium ion secondary battery that is being charged at a predetermined current rate higher than or equal to a predetermined charging rate; obtaining a rate of increase in the measured voltage; and estimating a state of charge of the lithium ion secondary battery based on the rate of increase in the measured voltage and first reference data. The first reference data are data including a correlation between a state of charge of a reference lithium ion secondary battery and a rate of increase in voltage of the reference lithium ion secondary battery when the reference lithium ion secondary battery is charged at the predetermined current rate.