A method of detecting a state-of-charge of a battery detects a current of a battery and a voltage of the battery, calculating a state-of-charge of the battery as a first state-of-charge based on an integration of the current of the battery thus detected, while calculating the state-of-charge of the battery as a second state-of-charge based on the voltage of the battery and calculating a synthetic state-of-charge obtained by taking a weighted mean of the first state-of-charge and the second state-of-charge as the state-of-charge of the battery, and weighting the weighted mean in order to increase weighting of the second state-of-charge in a region in which a capacity of the battery is increased and a region in which the capacity of the battery is reduced and to increase weighting of the first state-of-charge in other regions.