Abstract: In an ionization device of a mass spectrometer, an abnormality diagnosis device or control section switches a change-over switch without lighting up a filament, and reads a trap current and a total current supplied to the filament, by current/voltage converting sections. Then, by determining whether the trap current and the total current are zero or more, a trouble due to a contact between the filament or a trap electrode and an ionization chamber is found. When it is determined that a condition is normal, the filament is turned on and the feedback control is operated. Also, the trap current and the total current are respectively read, and in accordance with the values thereof, there is found a deficiency due to a shift of the attachment position of the filament or the trap electrode, or a burnout of the filament.