An input waveform follow-up AC power source system is shown, which can obtain a desired sinusoidal voltage or current wave continuously and with the same or a similar waveform from a constant power source, as well as being small in size and light in weight.The AC power source system, which provides a desired AC voltage or current from an AC power source, comprises a first AC switch provided between the input side and the output side and on-off operated in a predetermined cycle, and a second AC switch provided on the output side of the first AC switch and at a position to short-circuit the output side and on-off operated conversely to the first AC switch. A predetermined pause time is provided between the operations of the first and second AC switches. Each AC switch has two semiconductor elements, and diodes each connected between controlled terminals of and in opposite conduction polarity to each semiconductor element, like polarity controlled terminals of the two semiconductor elements being connected to each other, the same control signal being supplied to a control input terminal of each semiconductor element for on-off switching AC between the other controlled terminals of the semiconductor elements.