An A/D conversion device includes a counter for counting a clock signal fed from a clock signal generator, a D/A converter for converting a digital output of the counter into an analog value, a comparator for comparing an analog output of the D/A converter and an analog field current detection value fed from a current detecting resistor and outputting a result of comparison, and an up/down control circuit for switching the counter between count-up and count-down operations according to the result of comparison output from the comparator. The output of the counter is transmitted as a digital field current detection value to an external control unit which is an ECU of a vehicle via a communication interface.