PURPOSE: To highly precisely detect fuel concentration in purging air by providing a measuring part having mutually connected electrode pairs in a purging passage in parallel, and detecting an electrostatic capacity change.

CONSTITUTION: A measuring part 34a provided in a purging passage having mutually parallely connected electrode pairs Cs (Cs1-Csn) functioning as a capacitor. Oscillation is performed in a frequency corresponding to capacitance by an LC resonance circuit composed of a coil L and a capacitor C, a resonance wave is converted into a pulse signal with a transistor Tr1 and a resistance R3 and is output to a dividing circuit 103. In the dividing circuit 103, the frequency of the converted pulse signal is divided at a specified dividing ratio, and the divided pulse signal is output to an F/V conversion circuit 104. In the F/V conversion circuit 104, voltage Vo is output on the basis of the dividing frequency of an input pulse signal, and detection of capacitance is performed by the measurement of the voltage Vo.