First, second, and third electrodes are exposed to a fuel passage. The third electrode defines a first gap with the first electrode and defines a second gap with the second electrode. A property detection unit detects a property of fuel according to a summation of a first capacitance of the first gap and a second capacitance of the second gap. A correct-erroneous determination unit determines whether the property detected with the property detection unit is correct or erroneous according to a ratio of the summation and the first capacitance.