The power source apparatus is provided with a shunt resistor connected in series with batteries, and a current computation circuit that detects the voltage induced by current flow through the shunt resistor to compute battery current. The shunt resistor is provided with a pair of current flow terminals at two separated points on a metal plate connected in series with the batteries via connecting leads, and a pair of voltage detection terminals on a side of the metal plate between the pair of current flow terminals. Further, the shunt resistor has interval adjustment structures to adjust the distance (L) between attachment points where the connecting leads attach to the pair of current flow terminals. The distance (L) between lead attachment points is adjusted with the interval adjustment structures to finely adjust the voltage induced at the voltage detection terminals due to current flow between the two attachment points.