Foreign substances present in a sample chamber are attached to or drawn close to an objective lens and an electrode disposed close to the objective lens by applying a higher magnetic field than when normally used to the objective lens and applying a higher electric field than when normally used to the electrode disposed close to the objective lens. A stage is moved such that the center of an optical axis is located directly above a dedicated stand capable of applying voltage, the magnetic field of the objective lens is turned off, and then the potential difference between the electrode disposed close to the objective lens and an electrode disposed close to the sage is periodically maximized and minimized to thereby forcibly drop the foreign substances onto the dedicated stand capable of applying voltage.