PURPOSE: To prevent temporary enriching of air-fuel ratio at the time of changing the number of operative cylinders and to thereby prevent temporary increase or decrease of noxious components contained in exhaust gas, by providing a cylinder- number controlling circuit which is adapted for interrupting or removing interruption of fuel supply signal with delay of a certain time period at the time of changing the number of operative cylinders.

CONSTITUTION: In cylinder-number controlling circuit 13, there is provided a timeing delaying circuit 20 for controlling the injection timing of fuel injection signals to inoperative cylinders #4W#6. This delay circuit 20 judges the load from the pulse width of fuel injection signals and affords at a predetermined timing to injection-signal interrupting circuit 22 a cylinder-number signal of load judging circuit 21 which produces output "0" for instructing partial cylinder operation signal when the load is low and produces output "1" instructing full-cylinder operation signal in other cases. That is, in changing the mode of engine operation from its full-cylinder operation to partial-cylinder operation, fuel supply is interrupted after supply of fresh air to inoperative cylinders is interrupted, and a signal for doubling the pulse width of injection signal is produced.