In this paper, the author experimentally investigated the pressure drop due to friction for the flow of water containing bubbles in a non-heated downcomer. First, the pressure drop due to friction and dryness fraction for the parallel downward flow of the mixtures of air and water were measured in a vertical 1500 mm long and 19 mm inside diameter glass pipe. Next, the ratios of the coefficient of friction in high pressure regions were extensively computed from the results of the above experiment. Finally, in case we used nonheated downcomer, it was proposed that we might adopt 0.03 as an approximate value of the coefficient of friction.