Abstract Electron cyclotron heating (ECH) is one of the auxiliary heating methods employed in many magnetically confined fusion devices. In the Large Helical Device (LHD), the ECH system - including high-power gyrotrons, transmission lines, and mirror antennas – has played an important role in improving plasma parameters and realizing a variety of plasma experiments. Highly efficient transmission lines capable of withstanding high-power microwaves are essential to deliver high-power microwaves emitted from gyrotrons. In addition, mirror antennas are important for ensuring that the ECH power is absorbed at the intended location. In this paper, the ECH transmission system, focusing on the associated components used in high-power microwave transmission lines and various mirror antennas prepared for the LHD experiment, is reviewed.