The present invention relates to an offset apparatus for an NC machine tool, that can achieve high accuracy machining by offsetting the amount of operation of a motion mechanism in accordance with the analysis result of the behavior of the NC machine tool. The offset apparatus 1 comprises: a model analyzing data base 12 in which three dimensional model data of the motion mechanism 2 and condition data for performance analysis are stored; a model analyzing section 13 which analyzes the performance of the motion mechanism 2; a data accumulating section 14 which stores analyzed performance analysis data; an offset amount computing section 15 which computes an offset amount to be applied to a commanded operation amount on the basis of the performance analysis data; and an offset executing section 17 which offsets the amount of operation of the motion mechanism 2 on the basis of the offset amount.