Abstract: This paper proposes a topology optimization method and rotor structure designs for a permanent magnet synchronous generator based on this method. These described designs enable high output power and less permanent magnet volume. The topology optimization method is a genetic algorithm that considers the clusters and cleaning procedure of material. The optimized rotor has a structure in which the magnet is moved in the direction of rotation, and the output power of the optimized rotor is larger than that of the Prius type generator, in spite of less PM volume.