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 a 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.