1. A thermal displacement correcting apparatus for a machine tool having sets of specifications that differ from one another due to at least some estimated elements being different, comprising: a correction amount estimating portion that calculates a thermal displacement correction amount of the machine tool of at least one of the sets of specifications; and temperature sensors that measure a temperature of the estimated elements, wherein the correction amount estimating portion stores a collection of types of the temperature sensors corresponding to each of all of the estimated elements as a temperature sensor selection database, is able to read machine information according to the at least one of the sets of specifications, selects the type of the temperature sensors corresponding to the estimated elements belonging to the at least one of the sets of specifications from the temperature sensor selection database based on the machine information, calculates an estimated thermal displacement of each of the estimated elements based on the temperatures of the estimated elements ascertained by the temperature sensors of the selected type, and calculates a thermal displacement correction amount of the machine tool of the at least one of the sets of specifications by combining the estimated thermal displacements.