Horizontally steerable ECRH antenna system of central cell on GAMMA 10 is developed for efficient electron heating. The steering mirror antenna can shift horizontally absorption position at the resonance surface in the range of -7.3 ~ +7.3 mm from standard position without significant power reduction which is examined with both calculation and cold power test. As a result of the plasma experiments of the antenna position scan, it is found the horizontally optimized position for ECRH. The spilled RF from mirror(the stray RF) seems to deteriorate the hot ion confinement. The newly installed efficient antenna system which has higher transmission efficiency of ~90% has shown improved heating performance.