A single air intake port 32 is connected to a principal surface 29 of a combustion chamber defining portion 105 of a cylinder head 22. A first end 372a of a valve shaft 372 of an air intake valve 37 is connected to a valve head. An injection nozzle 47b of an injector 47 injects the fuel in the form of mist toward the combustion chamber B4 through the air intake port 32. At least while idling, assist air is led to a space near the injection nozzle 47b. As viewed in the cylinder axis direction, the first end 372a of the valve shaft 372 is arranged between the center axis A4 of the injection nozzle 47b and an igniter 115 of an ignition plug 104. This improves fuel combustion efficiency by promoting atomization of fuel.