PURPOSE: To always detect the composition ratio of a mixed fuel with high accuracy by detecting the heat conductivity characteristics of the fuel based on the elapsed time during which the resistance value of a thin-film resistance changes by a prescribed quantity.

CONSTITUTION: Signals from a monostable multivibrator 22 generated when an oscillator 21 rises are inputted to a transistor (Tr) 24 and, when the Tr 24 is turned off, a prescribed voltage relying on the ratio of the resistance value R of a thin-film resistance constituting a fuel sensor 6 to the resistance value R0 of a resistance 25 is applied across the sensor 6. When the Tr 24 is turned on, on the other hand, no voltage is applied across the sensor 6. Moreover, a voltage from a battery power source Vcc is inputted to the input terminal 27a of a comparator 27 in corresponding to the resistance ratio between resistances 26A and 26B and the voltage applied across the sensor 6 is inputted to the other input terminal 27b of the comparator 27. Then the comparator 27 compares the two input signals with each other about the magnitude. At an AND circuit 28 the AND of the output signal of the comparator 27 and the inverted output signal of the monostable multivibrator 22 is taken and a result is outputted.