To accurately measure a work temperature while following a temperature change, and to temperature-correct precisely a measured length of a measuring portion in a work in response to the work temperature.

The work is fixed in one portion onto a jig of a three-dimensional measuring instrument to allow linear expansion. Coordinate values of X-, Y-, Z-axes in at least two reference portions of the work are measured based on detection signals of a probe from the three-dimensional measuring instrument, and a work coordinate system is set based on the coordinate values. Coordinate values of X-, Y-, Z-axes are read based on a detection signal of the probe as to the measuring portion of the work, and a surface temperature of the work is measured by a radiation thermometer. The measuring length in the work coordinate system in the measuring portion is computed based on the coordinate values. The measuring length in the measuring portion is temperature-corrected into a measured length in the reference portion in the work at the reference temperature, based on a difference between the work temperature and the reference temperature.