Abstract: An alcohol concentration detecting device is disclosed, which comprises an alcohol sensor of capacitance type having a probe portion which is to be submerged in an alcohol-mixed fuel, the alcohol sensor issuing an output signal which varies in accordance with the concentration of alcohol in the alcohol-mixed fuel; a first oscillating circuit which receives the output signal of the alcohol sensor and issues a variable pulse signal whose duty ratio is variable in accordance with the output signal; a second oscillating circuit which issues a reference pulse signal, the reference pulse signal being the same as the pulse signal which is provided by the first oscillating circuit when there is no alcohol in the fuel; and an alcohol concentration detecting circuit which receives and compares the variable pulse signal and the reference pulse signal and issues an alcohol concentration representing signal which is variable in accordance with a difference in duty ratio between the variable pulse signal and the reference