The purpose of the present invention is to provide an overlay error measuring device for correcting a pattern displacement other than an overlay error to thereby achieve high-precision overlay error measurement. To accomplish the abovementioned purpose, the present invention proposes an overlay error measuring device which measures a dimension between a plurality of patterns belonging to different layers using a signal obtained by a charged particle beam device, and when measuring the dimension, corrects an amount corresponding to a pattern shift due to an optical proximity effect and measures the dimension between the plurality of patterns.