PURPOSE: To protect a load circuit and the internal circuit of a detector against breakdown or burning by checking a DC-DC invertor for abnormality at the time of starting.

CONSTITUTION: At the time of starting a DC-DC converter 2 employing a primary or a secondary battery as a power supply 1, the output terminal voltage characteristics thereof are detected through a voltage detection circuit 5. A system control circuit 6 compares the detected output terminal characteristics with a normal area data being stored as predictive output terminal voltage characteristics. When the characteristics deviate significantly from each other, a decision is made that the DC-DC converter is abnormal and the operation thereof is stopped immediately thus protecting a load circuit 7 and the internal circuit of the detector against breakdown or burning.