PURPOSE: To take in information on the degree of opening of a throttle valve in the low region, with fine resolution, through a device of low cost, by converting the signal of a throttle sensor into a frequency corresponding to the output voltage and by reading the frequency through a timer.

CONSTITUTION: When an internal combustion engine is working, a throttle sensor 1 generates an output voltage proportional to the degree of opening of a throttle valve. The output voltage is converted into a frequency signal by a V/F converter 3. The frequency signal is processed through a microcomputer in a central processing control unit 5 so that the period of the signal is read by an interval timer 4. In the control unit 5, a basic quantity of fuel is determined, according to a two-dimensional map, from rotational frequency information 8 and throttle valve opening degree information converted into the period. The basic quantity of fuel is compensated by intake air temperature information 9, water temperature information 10 and O2 sensor information 11 and applied from the timer 4 to determine the duration of opening of an injection solenoid valve 7.