A short-circuit detector has connected between positive and negative terminals of a DC power source a series connection including sequentially a first switch, a first detection resistor, a second detection resistor and a second switch, a third switch between a grounding terminal such as a vehicle chassis and a point between the first and second detection resistors and a voltage detector circuit connected in parallel with the first and second detection resistors. A control circuit is connected to the voltage detector circuit, serving to calculate a voltage value of the DC power source by switching on only the first and second switches, to calculate a first voltage drop value across the first detection resistor by switching on only the first and third switches, to calculate a second voltage drop value across the second detection resistor by switching on only the second and third switches and to determine presence or absence of short-circuited conditions at the positive and negative terminals of the DC power source on the basis of the calculated voltage value, the first voltage drop value and the second voltage drop value.