An electronic control unit for controlling a fuel injection amount of a two-cycle internal combustion engine with an exhaust control valve wherein the electronic control unit comprises an injection amount determination means for determining the fuel injection amount at various control conditions, a valve abnormality detection means for detecting an abnormality of operation of the exhaust control valve, and an injector driving means for driving an injector so as to inject fuel of the amount determined by the injection amount determination means from the injector. The injection amount determination means determines the injection amount so that when the valve abnormality detection means detects the abnormality of operation of the exhaust control valve, an air-fuel mixture is made in a rich state.