A state-of-charge detector device for detecting the state of charge of a secondary battery is provided. The device has a current detector that detects an electric current through the secondary battery, a voltage detector that detects a voltage between terminals of the secondary battery, and an internal resistance calculator that calculates an internal resistance of the secondary battery based on the detected current and the detected voltage. Furthermore, an internal resistance changing rate calculator is provided which calculates a rate of change in the internal resistance calculated, with respect to an amount of charge stored in the secondary battery. A state-of-charge determinator determines a state of charge of the secondary battery based on the rate of change in the internal resistance calculated.