Provided is a control device for an internal combustion engine, employing a microprocessor to control a load other than an ignition device, the control device being provided to an internal combustion engine in which is installed a magnet generator that has a magneto coil for successively generating, in association with revolution of the internal combustion engine, a first half wave voltage, a second half wave voltage of different polarity than the first half wave voltage, and a third half wave voltage of identical polarity to the first half wave voltage; and the magnet generator employing the second half wave voltage to drive the ignition device. The device is provided with an electricity storage element which draws excess power from the output that is output by the magnet generator for the purpose of driving the ignition device, and which is charged by the first and second half wave voltages, as well as being charged by the second half wave voltage as well at times that the internal combustion engine is in the exhaust stroke, in order to supply power to the load and to the microprocessor. The power source circuit is constituted to use the energy stored in this electricity storage element to generate power source voltage for presentation to the microprocessor and to the load other than an ignition device.