To provide an ionizer wherein an air flow path in a hollow casing is formed so as to be pressure-resistant while ensuring an effective cross sectional area without becoming voluminous, and the flow path is ensured simply and easily. In an ionizer including a hollow casing that contains a high voltage power unit and a control unit inside, along the longitudinal direction of the casing, a plurality of electrode units each including needle electrodes and an air outlet for blowing out an air stream around the needle electrodes are arranged in a line. A part along the longitudinal direction of the hollow casing is formed of a synthetic resin, and a resin casing composed of the synthetic resin is integrally formed with a conduit of an air flow path extending in the longitudinal direction of the casing so that a part of the inner wall of the resin casing serves as a wall of the conduit and that the air flow path communicates with the air outlet.