PURPOSE: To display the state of charge or discharge of a battery by a method wherein one or the other of a pair of light emitting devices is lighted by the current direction detected by a current direction detecting means.

CONSTITUTION: Between the battery 1 and a generator 5 there is provided a current direction detecting means composed of a current detecting resistor 7, resistance-type potential dividers 8, 9, a transistor 11, comparators 12, 13. When an engine begins to rotate, a contact point W of a voltage regulator 6 is connected to a contact point B to release the base of the transistor 11 from the common potential. Accordingly, the transistor 11 is truned on and the voltages at voltage dividing point A, B make almost the half of the voltage of the battery 1. At this time when the current passing through the resistor 7 runs from the battery 1 toward a boad 4, the voltage at terminal T1 becomes higher than the voltage at terminal T2. Accordingly, since the voltage at an inverted input terminal is higher than the input of the comparator 12, the output makes a low potential and a light emitting device 14 is not lighted. On the contrary, since the non-inverted input terminal of the comparator 13 is higher in voltage than the inverted input terminal, the output of a position 3 makes a high potential and a light emitting device 15 is lighted.