A detector device of the present invention detects densities of components, such as gasoline and ethanol, contained in mixture fuel even when some water is included in the mixture fuel. The detector device includes a sensor having a pair or electrodes, an electronic device for calculating the densities and a memory device for storing permittivities of pure components including water measured beforehand. Alternating current having two different frequencies f1, f2 is applied to the pair of electrodes immersed in the mixture fuel to detect the permittivities of the mixture fuel under f1 and f2. The two frequencies, f1 and f2, are so chosen that the premittivities of gasoline and ethanol show no change between f1 and f2, while the permittivitiy of water shows a substantial difference between f1 and f2. The electronic device calculates the densities of the components based on permittivities of the mixture fuel detected by the sensor and those of components stored in the memory device.