Plasma breakdown for an electric field of (loop voltage of 1.7 V) was demonstrated in JT-60U using helium prefilling gas and LHRF heating. Runaway electrons were not generated even for the low prefilling pressure of required for plasma breakdown at low loop voltage. Preionization was not obtained by LHRF heating, but the time taken to pass the radiation barrier was significantly shortened and a subsequent plasma-current ramp-up at a plasma current of < 30 kA was successfully obtained. As a result, a plasma-current start-up of with a loop voltage of was demonstrated when the plasma current was 35 kA to 1 MA.