A spherical aberration corrector is offered which permits a correction of deviation of the circularity of at least one of an image and a diffraction pattern and a correction of on-axis aberrations to be carried out independently. The spherical aberration corrector (100) is for use with a charged particle beam instrument (1) for obtaining the image and the diffraction pattern and has a hexapole field generating portion (110) for producing plural stages of hexapole fields, an octopole field superimposing portion (120) for superimposing an octopole on at least one of the plural stages of hexapole fields to correct deviation of the circularity of at least one of the image and diffraction pattern, and a deflection portion (130) for deflecting a charged particle beam.