The present invention provides a membrane-electrode assembly which includes: at least one rod-form or tubular electrode a tubular diaphragm disposed around the periphery of the electrode and a wire-form counter electrode disposed around the periphery of the diaphragm, the diaphragm being fixed to the rod-form or tubular electrode with the wire-form counter electrode to thereby form an electrode chamber having a gas/liquid passage between the diaphragm and the rod-form or tubular electrode.