PURPOSE: To achieve the high-accuracy control of air-fuel ratio, by fitting a pair of conductive members so as to face to the inner wall of an intake passage, and applying an AC voltage to detect the fuel flow rate along wall face from the quantity of passed electricity.

CONSTITUTION: An insulating member 2 is screwed into the wall of the intake manifold 1 in the vicinity of an engine intake port. In the insulating member 2, a pair of platelike conductive members 4a, 4b are fitted in said member a so as to face to the inside of an intake passage 3 and to be opposite to each other at a prescribed space. The conductive member 4a on one side is connected to an AC power supply 5, and the conductive member 4b on the other side is connected to the ground. When a given ac voltage is applied between the conductive members 4a and 4b through a power supply 5, a current value is measured by an ac ammeter 6, and by the change of this current value, actual liquid fuel flow rate along wall face flowing between the conductive members 4a and 4b is calculated. Accordingly, by correcting the difference between the fuel flow rate at the time of steady state and the actual fuel flow rate, the fuel delay at the time of acceleration/deceleration can be corrected so that an air-fuel ratio can approach its proper value.