The present invention has the object of providing a charged particle beam irradiation method ideal for reducing the focus offset, magnification fluctuation and measurement length error in charged particle beam devices.To achieve these objects, a method is disclosed in the invention for measuring the electrical potential distribution on the sample with a static electrometer while loaded by a loader mechanism. Another method is disclosed for measuring the local electrical charge at specified points on the sample, and isolating and measuring the wide area electrostatic charge quantity from those local electrostatic charges. Yet another method is disclosed for correcting the measurement length value or magnification based on fluctuations found by measuring the amount of electrostatic charge at the specified points under at least two charged particle optical conditions, and then using a charged particle beam to measure fluctuations in measurement dimensions occurring due to fluctuations in the electrostatic charge at the specified locations.