To prevent the stall feeling caused by a lean air-fuel ratio by setting a delay time when lean air-fuel ratio control is switched to rich air-fuel ratio control, and selecting the lean air-fuel ratio control when the shift gear of a transmission is located at the prescribed position and the set delay time elapses.

An electronic control device 6 receiving the output signals of an intake pressure sensor 13, an engine speed sensor 14, a throttle sensor 16, and an air-fuel ratio sensor 21 switches lean air-fuel ratio control and rich air-fuel ratio control including power air-fuel ratio control and stoichiometric air-fuel ratio control in response to the operation state. The electronic control device 6 sets a prescribed delay time when the lean air-fuel ratio control is switched to the rich air-fuel ratio control, and it judges the shift gear position of a transmission. If the throttle opening is in the lean air-fuel ratio control when the judged shift gear position is the prescribed position and the set delay time elapses, it selects the lean air-fuel ratio control.