PURPOSE: To aim at improvement in the accuracy of the fuel injection quantity when an engine is in its decelerating operation by decreasing the then frequency of injection to less than that at the time of its normal operation when the engine is judged to be in its decelerating operation, while increasing the quantity of injection per one time of injection frequency.

CONSTITUTION: There is provided a decelerating operating state judging means B for judging the presence of a decelerating operating state according to output signals from a detecting means A for detecting the decelerating operating state of an engine. In addition, when an engine is judged to be in its non-decelerating operation, an injection quantity at the time of its normal operation is set up by a normal injection quantity setting means C, and at the same time, an injection frequency per a fixed engine speed is set up by a normal injection frequency setting means D. On the other hand, when an engine is judged to be in its decelerating operation, the then injection frequency is set up by a decelerating injection frequency setting means E so that it is decreased to less than that at the time of its normal operation, and at the same time, the injection quantity is set up by a decelerating injection quantity setting means F in such a manner that the quantity of injection per one time of injection frequency is increased by the amount decreased due to the reduction of injection frequency by way of making the air-fuel ratio to be leaner than that at the time of its normal operation, and then, a fuel injection value G is controlled in response to those outputs.