The principle of a homopolar machine (acyclic machine or unipolar machine) has been known since the days of Faraday, and the machine features a direct current machine of low voltage and large current. The construction of this machine is simple and solid, and its application was long awaited. In the 1970s, the practical large superconducting magnet became available on the market, and the merit of homopolar machines it provided was reconsidered, bringing about many developments in various fields. The technology of an efficient and stable large current collection from the high-speed rotating armature was pursued with a carbon brush or liquid metal. Today, homopolar machines are worth being again reconsidered, by a result of the advent of the easy handling conduction cooling magnet and the technological development of the high temperature superconducting magnet. In this paper, the development of a 3, 000kW superconducting homopolar generator, the principle of the homopolar machine, an equivalent circuit, a collector, characteristics, development, and application are described.