A concentrated vegetable and/or fruit product is obtained by reverse osmosis by a method of first preparing a vegetable and/or fruit product including seeds, epidermis and/or sarcocarp as a solid component, causing it to flow down under a pressured condition to a concentration unit having an inlet, an outlet and tubular membrane modules connected in series such that it is supplied to the tubular membrane modules through a single-axis eccentric screw pump attached to the inlet and discharging a concentrated product from the tubular membrane modules through another single-axis eccentric screw pump which is attached to the outlet.