To provide a thermal displacement correcting apparatus and method of a machine tool, capable of precisely and easily setting or operation even though the nunmber of specifications is large.

The thermal displacement correcting apparatus includes a correction amount estimating device 13 and parameter automatic selecting device 14, which estimate a thermal displacement correction amount of the machine tool of each set of specifications, in a machine tool having two different sets of specifications that differ due to the presence of a scale 2 and different methods of measuring the lengths of a bed 1 and a table 5. The correction amount estimating device 13 and parameter automatic selecting device 14 include: storing a collection of parameters for an estimated thermal displacement calculation corresponding to each of the bed 1, the scale 2, the table 5, and a workpiece 6, as a database; selecting the parameters belonging to the set of specifications from the database based on machine information according to each set of specifications; calculating estimated thermal displacements of the bed 1, the table 5, and the workpiece 6, or also the scale 2, according to the selected parameters; and combining the estimated thermal displacements.