1. A method for detecting abnormalities of a temperature sensor in a machine tool in which a plurality of temperature sensors are arranged at different parts of the machine tool, the method comprising the steps of: preliminarily grouping the temperature sensors into one or a plurality of predetermined attachment areas of the machine tool at which temperatures vary in a similar manner; setting a limit value of variations of detected temperatures of each group of the plurality of temperature sensors belonging to the same group in the predetermined attachment areas; detecting temperatures of each part of the machine tool by the plurality of sensors; grouping the detected plurality of temperatures in accordance with the corresponding group of temperature sensors; obtaining variations of the detected temperatures in the same temperature group, respectively; comparing each of the variations with a corresponding preset limit value of each group, respectively; and determining the temperature sensors associated with the variation to be abnormal when one of the variations exceeds the limit value.