A cycloconverter system, small and simple, has been developed in the digital laboratory. In the system, the microprocessor is used effectively, and unnecessary hardwares are eliminated. The system has been highly improved in its stability and reliability by digital control circuitry. As the system is small and simple, it is easy to check errors and maintain the system. Besides all of the functions of the system are controlled by software, so that we can give the system flexibility for control functions. If we need other functions, we only have to change the program. A similar system was developed in 1978. It replaced conventional analog control circuitry with digital circuitry. But it used so many hardwares that it was hard to check and maintain the system. On the contrary, the system, developed in 1979, requires very little hardwares. So we have made it possible to apply the cycloconverter to practical fields in future.