PURPOSE: To accurately detect the volume ratio by excluding the influence of a shunt resistor due to water contamination and preventing the influence of electromagnetic interference in a fuel sensor system for detecting the volume ratio of methanol mixed with gasoline by capacity detection.

CONSTITUTION: The capacity component 14 of fuel to be detected by a sensor 12 and a capacitor 30 having similar value as that of a shunt resistor 13 and a resistor 31 are provided in a reference (dummy) sensor 28. These two sensors 12, 28 are driven by a pulse from a square wave oscillator 19 via capacitors 20, 29. The outputs from the two sensors are input to AM detectors 17, 32. Then, the difference of peak-detected outputs are obtained by a differential buffer 18, and supplied to an engine control module (ECM). Since the difference of the outputs of the detectors 17, 32 is obtained by the buffer 18, the influence of the shut resistor is canceled, and even if the frequency of the oscillator is not raised, the capacity component can be detected and hence it can be accurately detected without generating electromagnetic interference.