A 170 GHz gyrotron has been developed at JAEA, which has achieved operation of 1 MW/800 s and up to 55% efficiency. This is the first demonstration of a gyrotron achieving and even exceeding the ITER operating requirements of 1 MW/500 s and 50% efficiency. In addition the gyrotron demonstrated operation for 1 h with an output power of 0.6 MW. The oscillation was stable with all cooling water temperatures and vacuum pressure reaching equilibrium conditions during the pulse length at either power level. The successful operation was aided by a very low level of stray radiation (∼2% of the output power), which contributed to fast conditioning and stable operation. The output power from the gyrotron was coupled into an ITER sized corrugated waveguide (ϕ = 63.5 mm) via a matching optics unit with a total of 0.97 MW transmitted to the dummy load after two miter bends and ∼7 m of a waveguide without any trouble. These results give an encouraging outlook for the success of the ITER electron cyclotron heating and current drive system.