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Reduction of water loss from leafy vegetables is important to keep freshness. The hydraulic conductivity coefficient of cell membrane (Lp) is indispensable to analyze the movement of water from cells which leads to wilting and low metabolic activity. To measure Lp, extra-cellular solution must be replaced quickly to apply osmotic stress to cells, however, the conventional methods take a relatively long times which leads to noticeable errors. We therefore developed a new method to measure Lp, based on the principle that two liquids flow in laminar flow though relatively narrow space, for example, a tube having a small inner diameter. A micro-channel having a diameter of 1.4 mm was designed. The micro-channel has two inlets and the flow rate of solution at each inlet can be used to determine the volume of each solution occupying the micro-channel. Hence, the extra-cellular solution of protoplast fixed in a micro-channel can be changed instantaneously by the changing the flow rate of solution. The Lp of spinach leaf protoplasts was measured by both the developed method and a conventional method. Lp measured by the conventional method was 0.03 to 0.07x10-12[m/s/Pa] and that measured by the newly developed method was 0.076x10-12[m/s/Pa]. The difference in values measured by both methods was examined by error estimation and the value obtained by the new method was judged valid. With this method, the time delay for replacing of extra-cellular solution was negligible and Lp was obtained without the error caused by the time for replacement.