Abstract: To simplify the structure of a double (two and four) cycle internal combustion engine, the engine comprises a cylinder formed with a scavenging port and an exhaust port, a piston, a fuel injection valve, a fuel ignition plug, an air flowmeter, a crank angle sensor, and a controller for switching the engine operation from 2 cycle mode to 4 cycle mode or vice versa according to engine operating condition (engine speed and load) and for activating the fuel injection valve and the fuel ignition plug every engine revolution in two cycle mode and every two engine revolutions in four cycle mode. Since the engine operation mode can be switched by only changing the fuel injection and ignition timings through the controller, the intake and exhaust valves will not be complicated without need of any additional valve actuating mechanisms.