To provide an electron beam length measuring apparatus and length measuring method capable of suitably preventing a measuring object from being contaminated and shortening the time required for length measurement processing.

This electron beam length measuring apparatus is provided with an electron gun 16 for generating an electron beam, a deflector 26 for deflecting an electron beam, a stage 14 part for placing a length measuring object 12, a detector 36 for detecting electrons scattered due to the electron beam, a storage part 50 for storing design position information for specifying the design position where a designated length measuring part is to be existent in the length measuring object, and a length measurement control part 58 for scanning an electron beam to apply the electron beam to the design position where a designated length measuring part is to be existent by the deflector 26, and measuring the length of the length measuring part according to the change state of electrons sequentially detected by the detector 36 during scanning of the electron beam.