Parts-attaching grooves are formed at both side surfaces of an ionizer main body, and an auxiliary member for extending parts is detachably latched to the parts-attaching grooves. Parts, such as sensors, filters, and so forth are extended by means of the auxiliary member. The aforementioned parts attaching grooves are constructed such that a lower side groove wall is slanting in a gradually outside-down manner toward a groove opening side, and further the auxiliary member includes a projection edge for latching, at a tip end of left and right side walls, and the auxiliary member is attached to the aforementioned ionizer main body by means of elastically latching the projection edge to the aforementioned parts-attaching groove.