A power supply unit comprises a battery, current and voltage detecting portions, and a remaining capacity computing portion. The remaining capacity computing portion comprises a first computing portion computing a first open circuit terminal voltage based on current and voltage values from the current and voltage detecting portions, and computing a first remaining capacity based on the first open circuit terminal voltage, a second computing portion computing a second open circuit terminal voltage based on the current and voltage values, an ohmic resistance voltage drop and a polarization resistance voltage drop, and computing a second remaining capacity based on the second open circuit terminal voltage, a third computing portion calculating a third remaining capacity based on an integrated value by integrating the current, and a main computing portion computing a real remaining capacity of the battery based on the first and/or third remaining capacity, and the second remaining capacity.