Abstract: A battery tester is formed at an AC constant-current generator for applying an AC current for measurement to a battery BT under test via a coupling capacitor, an internal resistance detecting device for detecting the internal resistance of the battery BT according to a voltage signal therefrom when the AC current is applied, a voltage detecting device for detecting the terminal voltage across the battery, and a deterioration judging unit for judging the deterioration of the battery from the internal resistance and terminal voltage detected by the internal resistance detecting device and the voltage detecting device. The internal resistance and terminal voltage of the battery are measured simultaneously, and deterioration of the battery is judged from the measurement data.