We successfully developed a Gyrotron HS-III (28-GHz/400-kW/5-s) for electron cyclotron resonance heating in a spherical tokamak EXL-50, which operated in the TE8,3 mode. An internal quasi-optical-mode converter was designed to separate the spent electron beam from the outgoing power and convert the operating mode to a fundamental Gaussian wave beam. The particle swarm optimization algorithm was used to reduce diffraction loss from the launcher in such a low-frequency band. This enabled us to optimize the perturbation distribution of the launcher and beam waist of the target function to obtain the largest possible scalar correlation coefficient. The launcher was followed by three mirrors: one quasi-parabolic mirror and two phase-correcting mirrors. These were also optimized based on the scalar diffraction theory and Katsenelenbaum–Semenov algorithm to allow us to change the beam direction and increase the purity of the Gaussian output beam. Simulation results revealed a transmission efficiency of 96% and vector Gaussian mode purity of 99.8% at the window plane.