To stably control a working machine engine even in a severe condition by providing an air-fuel ratio control means for determining a value of air-fuel ratio corresponding to the operating state and a valve opening sensor for detecting the opening of a throttle valve.

This device is provided with a throttle valve 5 for controlling the inflow quantity of air to a combustion chamber 1, a mechanical governor 6 for controlling the opening of the throttle valve 5, and a valve opening sensor 8 for substantially detecting the opening of the throttle valve 5. The mechanical governor 6 drives the throttle valve 5 in closing direction by a governor force GF. An injection setting part 32 calculates the intake quantity per one rotation of an engine on the basis of the opening of the throttle valve 5 and the engine rotating speed and determines the fuel injection quantity. 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 in conformation to the operating state. According to this, the throttle valve can be controlled with good responsiveness, and also precisely and stably controlled.