A rectangular cylindrical drift cell is formed by bonding four resistive film layer members in which a resistive film layer having a uniform film thickness is formed on a surface of a plate member formed of an insulator of alumina or the like through a coating method or a vapor deposition. A predetermined voltage is applied from a drift voltage generator to the resistive film layers of both ends of the drift cell so that an accelerating electric field having a linear potential gradient is formed along a center axis (C) in a desolvation region and a drift region inside the drift cell. Ions are drifted by the accelerating electric field and are separated in response to the ion mobility.
