Each of first to fourth needle-like electrodes is arranged such that a direction of extension thereof is parallel, and generates ions by discharge. Through a space, a gas for conveying the ions generated by the first to fourth needle-like electrodes flows. Needle tips of the first needle-like electrode and the second needle-like electrode protrude from a first wall surface that forms the space, are spaced apart from each other, and are arranged in line in the space. Needle tips of the third needle-like electrode and the fourth needle-like electrode protrude from a second wall surface that forms the space and faces the first wall surface, are spaced apart from each other, and are arranged in line in the space. The first needle-like electrode and the fourth needle-like electrode generate positive ions, and the second needle-like electrode and the third needle-like electrode generate negative ions.
