Disclosed is an apparatus for focusing a charged particle beam onto a speciman, which comprises a scanner for scanning a pre-formed standard pattern with a charged particle beam, a converging unit capable of converging the charged particle beam onto a specimen, a detector for detecting secondary charged particles emitted as a result of scanning the standard pattern by the scanner, a circuit for deriving a standard frequency component determined by the period of scanning with the charged particle beam and the shape of the portion of the standard pattern in the region being scanned with the charged particle beam, and a circuit cooperating with the converging unit for finding the maximum value of the amplitude of the standard frequency component thereby identifying attainment of focusing of the charged particle beam with high accuracy.