This paper shows that the design improvement of the ECH antenna system for high efficient heating of GAMMA 10 tandem mirror plasma. The previous ECH antenna was not large enough to cover the microwave because it was limited by the port size. New antenna system clears this limit and is enlarged as large as possible. And its size becomes twice, which enables to inject the microwave with the efficiency up to 95%. It is about 20% increase as compared with the previous system.