NOVELTY - The power supply device has an earth-leakage detection circuit (1) that detects earth-leakage resistance of an assembled battery (2) formed by connecting a capacitor (8) between the high voltage line. The earth-leakage detection circuit is connected in series through an earth-leakage detection switch (10). A voltage detection circuit (4) detects the earth-leakage voltage of the both ends of earth-leakage detection resistance (5) within a preset period. An arithmetic circuit (7) calculates earth-leakage resistance value from the earth-leakage voltage detected by a predetermined time interval.    USE - Power supply device, particularly an optimal power supply device for supplying an electric power to an electric vehicle, such as a hybrid car and for detecting earth-leakage of an assembled battery. Can also be used for an electrical storage apparatus of offpeak electric power, a cell electric power generating apparatus and wind power generation.    ADVANTAGE - The power supply device has an earth-leakage detection circuit that detects earth-leakage resistance of an assembled battery formed by connecting a capacitor between the high voltage line, and thus conserves the electric power, immediately detects the earth-leakage to the chassis earth from the high voltage line, rapidly and exactly detects the earth-leakage of the assembled battery in a simple formula without considering the electrostatic capacitance of the capacitor to the computing equation, correctly calculates the earth-leakage resistance value in the short time, and also reduces the time lag of the earth-leakage detection of the power supply device.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit diagram of an earth-leakage detection circuit of a power supply device. (Drawing includes non-English language text).    Earth-leakage detection circuit (1)    Assembled battery (2)    Voltage detection circuit (4)    Earth-leakage detection resistance (5)    Arithmetic circuit (7)    Capacitor (8)    Earth-leakage detection switch (10)