Charging and discharging lines for a rechargeable battery are arranged independently from each other, and as a protection circuit for the rechargeable battery, there are provided a nonreturn-type switch (for example, thermo-fuse with an internal heater) that is interposed in the charging line in series and interrupts the charging line by blowout and a semiconductor switch (for example, MOS-FET) that is interposed in the discharging line in series and electrically continues or interrupts the discharging line. The switch control circuit turns off the semiconductor switch element when detecting overdischarge of the rechargeable battery, and blows out the nonreturn-type switch when detecting overcharge of the rechargeable battery or a malfunction of the semiconductor switch element.