A fluid-property detection device for detecting a property of a detection target fluid contained in an external device, the fluid-property detection device comprising: a tubular outer electrode; an inner electrode provided inside the outer electrode; a computing unit configured to compute a property value of the detection target fluid based on an electrical property between the outer electrode and the inner electrode; a housing configured to be attached to the external device, the housing being configured to hold the outer electrode and the inner electrode; an outer insulating member provided between the housing and the outer electrode, the outer insulating member being configured to insulate between the housing and the outer electrode and being configured to define a position of the outer electrode with respect to the housing; and an inner insulating member provided between the outer electrode and the inner electrode, the inner insulating member being configured to insulate between the outer electrode and the inner electrode, being configured to insulate between the housing and the inner electrode, and being configured to define a position of the inner electrode with respect to the housing, wherein the outer electrode and the inner electrode respectively have tip end portions, the tip end portions projecting out from the housing and being configured to expose to the detection target fluid contained in the external device, the tip end portions of the outer electrode and the inner electrode are covered by an electrode cover attached to the housing, and the electrode cover is provided with a cover through hole through which an inside and an outside of the electrode cover are communicated, characterized in that the outer electrode is provided with an electrode through hole through which an inside and an outside of the outer electrode are communicated, the electrode through hole and the cover through hole are positioned on a line perpendicular to a center axis of the outer electrode. a tubular outer electrode; an inner electrode provided inside the outer electrode; a computing unit configured to compute a property value of the detection target fluid based on an electrical property between the outer electrode and the inner electrode; a housing configured to be attached to the external device, the housing being configured to hold the outer electrode and the inner electrode; an outer insulating member provided between the housing and the outer electrode, the outer insulating member being configured to insulate between the housing and the outer electrode and being configured to define a position of the outer electrode with respect to the housing; and an inner insulating member provided between the outer electrode and the inner electrode, the inner insulating member being configured to insulate between the outer electrode and the inner electrode, being configured to insulate between the housing and the inner electrode, and being configured to define a position of the inner electrode with respect to the housing, wherein the outer electrode and the inner electrode respectively have tip end portions, the tip end portions projecting out from the housing and being configured to expose to the detection target fluid contained in the external device, the tip end portions of the outer electrode and the inner electrode are covered by an electrode cover attached to the housing, and the electrode cover is provided with a cover through hole through which an inside and an outside of the electrode cover are communicated, characterized in that the outer electrode is provided with an electrode through hole through which an inside and an outside of the outer electrode are communicated, the electrode through hole and the cover through hole are positioned on a line perpendicular to a center axis of the outer electrode.