PURPOSE: To prevent the malfunction of a relay by applying the driving current to the relay after detecting that secondary power sources generated by a primary power source have risen.

CONSTITUTION: When secondary power sources E21 and E22 generated by a primary power source E1 rise, the base voltage is applied to a transistor TR1 by resistors R1 and R2 of a secondary power source detecting means 20, and TR1 is excited. When TR1 is kept off, TR2 of a primary power source control means 30 is set off because its emitter and base are kept at the same potential. When secondary power sources E21and E22 rise and TR1 is turned on, a current flows through resistors R3 and R4, the base voltage due to the voltage drop across the resistor R3 is generated, and TR2 is turned on. When a control signal is inputted to a relay driving means 40, polarity is reversed by an inverter INV1, TR3 is turned on, and a relay 10 is turned on.