Transmission system for the GAMMA 10 central-cell ECRH was newly constructed. This system was designed to maximize the calculated Poynting flux reaching the resonance surface. Performance test of main components of the transmission system was carried out with a low power source. The power transmission rates of the taper and the vacuum window were larger than 97 %. Microwave power as high as 88 % of that radiated from the open end of the corrugated waveguide was transmitted to the resonance surface with the designed mirror. This value agrees well with the value expected in our design.