In Large Helical Device, a power transmission line consists of corrugated waveguides with inner diameter of 88.9 mm for electron cyclotron resonance heating is evacuated in order to transmit higher power by reducing a possibility of electrical breakdown in the line. Some characteristics of such transmission lines, vacuum pumping system and comparison of pressure distribution simulated and gauged along the transmission line are described. An effect of vacuum pumping for higher power transmission is demonstrated.