An apparatus capable of improving image quality by making it possible to suck specimens of different sizes electrostatically, and uniformalizing an electric field of a specimen edge portion, while suppressing increase in prime cost is provided. Specimen holding means is an electrostatic chuck, a master flat plane part surrounding a specimen of the largest size of specimen sizes, and an opening surrounding a specimen size except for the largest specimen size are included at an outer peripheral portion of the electrostatic chuck, a dummy specimen attachable to and detachable from the electrostatic chuck is included, and at a time of switching the specimen size, a dummy specimen is selected (or may be prevented from being used).