A solution containing nonvolatile salts is pumped from a pump to an electrospray nebulization probe in the LC/MS interface, and spouted out from a tip of the probe into an atmospheric pressure environment in a form of fine liquid droplets having charges. The sample ions contained in the droplets are deflected by a deflector and enter into a mass analysis portion through an ion sampling aperture to be mass analyzed. On the other hand, the nonvolatile salts travel straight without being affected by the deflector, and collide against and are collected on a wall of a particle collector. The collected salts are precipitated in a form of crystals. The collected salts are washed away by spraying a particle washing solution from the washing nozzle. The above-described structure can provide an atmospheric pressure ionization mass spectrometer which can prevent effects of nonvolatile salts on the mass analysis without deteriorating the vacuum condition of the mass analysis portion by the preventing action.