1. Charged particle beam equipment comprising: a charged particle beam source; a charged particle optical system which illuminates a charged particle beam generated from the charged particle beam source on a specimen; a diffraction pattern detector which detects the intensity of a diffraction pattern generated from the specimen; a computer which reconstructs an object image from the intensity of the diffraction pattern; and an illumination adjustment system which determines the size of an illumination area of the charged particle beam illuminated on the specimen on the basis of the size of the object image reconstructed by the computer wherein the illumination adjustment system determines the size of an illumination area according to at least one of the wavelength λ of the charged particle beam, a camera length L at the time of obtaining the diffraction pattern, and the pixel size p of the diffraction pattern detector to determine the size of the illumination area, wherein the size of the illumination area is determined by the formula F=2·λ·L/p.