A method of controlling an engine by using a controller, the engine including a cylinder with a pentroof ceiling, air being introduced into the cylinder through an intake valve provided to the ceiling; an ignition device including a spark plug provided at or near the center axis of the cylinder; and a fuel injector provided at or near the center axis of the cylinder, the method comprising steps of: injecting main fuel by the fuel injector during one of an intake stroke and a compression stroke, and providing a mixture gas containing fuel and air inside the cylinder; applying by the ignition device a voltage between electrodes of the spark plug at a timing when the mixture gas is not ignited or not to be ignited, and detecting a parameter related to a current value of an electric-discharge channel generated between the electrodes; determining by the controller whether the detected parameter is within a range between a first threshold and a second threshold to determine a flowing state of a vortex inside the cylinder; injecting supplemental fuel by the fuel injector after the injecting the main fuel, and igniting the mixture gas by the ignition device using the spark plug after the injecting the supplemental fuel, when the parameter is determined to be outside the range; and igniting the mixture gas by the ignition device using the spark plug without injecting the supplemental fuel, when the parameter is determined to be within the range, characterized in that the injecting the supplemental fuel includes, when the parameter is determined to be below the first threshold, injecting the supplemental fuel at a first injection timing, and when the parameter is determined to be above the second threshold, injecting the supplemental fuel at a second injection timing retarded from the first injection timing. injecting main fuel by the fuel injector during one of an intake stroke and a compression stroke, and providing a mixture gas containing fuel and air inside the cylinder; applying by the ignition device a voltage between electrodes of the spark plug at a timing when the mixture gas is not ignited or not to be ignited, and detecting a parameter related to a current value of an electric-discharge channel generated between the electrodes; determining by the controller whether the detected parameter is within a range between a first threshold and a second threshold to determine a flowing state of a vortex inside the cylinder; injecting supplemental fuel by the fuel injector after the injecting the main fuel, and igniting the mixture gas by the ignition device using the spark plug after the injecting the supplemental fuel, when the parameter is determined to be outside the range; and igniting the mixture gas by the ignition device using the spark plug without injecting the supplemental fuel, when the parameter is determined to be within the range, characterized in that the injecting the supplemental fuel includes, when the parameter is determined to be below the first threshold, injecting the supplemental fuel at a first injection timing, and when the parameter is determined to be above the second threshold, injecting the supplemental fuel at a second injection timing retarded from the first injection timing.