In an ionizer, in which an electrode cartridge having a pair of electrodes is mounted in an electrode-attaching opening of a housing so as to be freely attached and detached by means of rotating the same around a center axial line, a drop-off prevention cover is attached to the housing. In the drop-off prevention cover, a fitting hole, to which the electrode cartridge is fitted, is formed, and by means of limiting a rotation of the electrode cartridge by the fitting hole, the electrode cartridge is prevented from dropping off.