A 2-MW, continuous wave, TE34,19 mode, 170-GHz coaxial cavity gyrotron for the International Thermonuclear Experimental Reactor (ITER) is under development within the European GYrotron Consortium (EGYC). This paper presents the improved design of the quasi-optical mode converter for this gyrotron. The simulation results show that with the improved quasi-optical mode converter, the fundamental Gaussian mode content of the rf beam can be enhanced to 99.1% at the output window plane and the stray radiation in the quasi-optical mode converter could be decreased to 1.2%.