A mass analyzer includes a mass analyzing magnet that applies a magnetic field to ions extracted from an ion source to deflect the ions, a mass analyzing slit that is provided downstream of the mass analyzing magnet and allows an ion of a desired ion species among the deflected ions to selectively pass, and a lens device that is provided between the mass analyzing magnet and the mass analyzing slit and applies a magnetic field and/or an electric field to the ion beam to adjust the convergence or divergence of a ion beam. The mass analyzer changes a focal point of the ion beam in a predetermined adjustable range between an upstream side and a downstream side of the mass analyzing slit with the lens device to adjust mass resolution.