PURPOSE: To prevent an engine from stalling as well as to make slow speed running performable, by installing an engine speed sensor detecting a slow speed running state in four-wheel drive and also an engine controlling computer controlling an air-fuel ratio to be set to the rich mixture side at the time of the slow speed running in this four-wheel drive.

CONSTITUTION: An engine 10 is provided with a water temperature sensor 20 detecting its water temperature and an engine speed sensor 22 detecting the engine speed. Each output of the water temperature sensor 20, the engine speed sensor 22, a shift position switch 24, a transfer switch 26, etc., is inputted into an engine controlling computer 30, and here injection time is found, then according to this injection time, a fuel injection valve 11 is driven for its opening or closing. An air-fuel ratio of the engine 10 is controlled by valve opening time of the fuel injection valve 11. Thus, at the time of slow speed running in four-wheel drive, combustion is yet more stabilized, preventing the engine from stalling, thus tenacious, smooth slow speed running is made performable.