To stably control a working machine engine even in a severe condition by providing an air-fuel ratio control means for correcting an injection quantity set by an injection quantity setting means and determining a value of air-fuel ratio corresponding to the operating state.

A fuel ignition plug 2 is provided facing a combustion chamber 1 of an engine, and a fuel injection device 4 is provided within an intake pipe 3. The opening of a throttle valve 5 for controlling the inflow quantity of air to the combustion chamber 1 is controlled by a mechanical governor 6. The processing performed by an injection setting part 32 comprises a step of predicting the air flow rate determined by the action of the mechanical governor 6 and a step of calculating the injection quantity and injection frequency in order achieve to the air-fuel ratio in steady operation on the basis of the predicted air flow rate and the engine rotating speed. An air-fuel ratio control part 33 judges a non-steady state requiring a special correction on the basis of the output of an operating state detecting part 25 and determines the value of air-fuel ratio. According to this, the throttle valve can be controlled with good responsiveness, and also precisely and stably controlled.