The 170 GHz gyrotron for ITER demonstrated the stable and steady state 1000s operation with the output power of 0.6 MW and the efficiency of 45% by depressed collector. The remarkable progress was obtained with optimization of a built-in launcher and mirrors for significant reduction of the stray radiation to ~2% of the output power, improvement of electron beam quality for high oscillation efficiency and control of constant beam current for stable oscillation. The result indicates a promising prospect for development of a 1 MW-CW-50%, 170 GHz ITER gyrotron.