To provide an apparatus and a method, for a length measurement by a charged beam, wherein the measurement error of a length measured result can be suppressed to a minimum.

In the charged-beam length measuring apparatus, a sample 3 in a prescribed scanning range is irradiated with a charged particle beam 4, and output data is formed on the basis of obtained scanning information. The same place of the sample 3 is measured in a plurality of numbers of times. When the error of at least one measured data from among a plurality of obtained measured data is large, the data whose error is large is deleted. When error data caused by a focus error or the like is deleted, the reliability of the apparatus can be enhanced.