PURPOSE: To speedily and correctly measure the ratio of a gas included in a fluid.

CONSTITUTION: Within the body 1 of the device disposed, placing an inflow port 2 down and an outflow port 3 up, a plurality of cylindrical electrodes 4, 7, the insides of which are to be used for measurement and the outsides of which are to be used for reference, are provided, a two-phase flow of gas- phase is allowed to flow in between the measurement electrodes 4, the liquid only is allowed to flow in between the reference electrodes 7 through a mesh filter 5 for restraining the inflow of bubbles, and a void ratio is measured from the difference of capacitances of both fluids under the same temperature condition.