Abstract: A deviation correcting method for a machine tool, which is capable of exactly correcting a deviation of a reference position of a tool with respect to a workpiece without repeatedly executing a measuring operation. The measuring operation includes providing a detector switch SW for detecting an operating end surface of the tool, obtaining a position z when the detector switch SW detects the operating end surface of the tool, and calculating a difference between the detected position z and a reference position z0 to obtain an amount of deviation A. This measuring operation is performed at least three times at an arbitrary time interval. Based on data of these actual measurements, a function f(Ti) of a curve representing a relationship between a time Ti and an amount of deviation A. Then, an amount of deviation A' at an arbitrary time point Ti is estimated using the function f(Ti) to correct a commanded position of the tool.