In a cycloconverter generator, AC power required by the load is detected and a desired speed of the engine is determined based on the required AC power, operation of an actuator is controlled such that the engine speed becomes equal to the desired engine speed, and the required AC power is generated by turning on switching elements (thyristors) such that a number of the phase signal within one period of frequency of the AC power, becomes equal to a number determined by the desired engine speed, thereby preventing undesirable increase in fuel consumption and noise by operating the engine at a speed corresponding to required AC power and enabling to generate stable AC power even when the engine speed changes abruptly.