A columnar rod reference member made of a material whose coefficient of thermal expansion is different from a flight tube is placed in contact with the tube. One end of the tube and o the reference member are fixed to each other with a fixture part. A distance measurement sensor measures the difference in length between the tube and the reference member whose lengths fluctuate due to a change in temperature. A displacement of the difference in length, as expressed by a proportion, is larger than that of the length of the flight tube. The difference in length is far smaller than the length of the flight tube. This improves detection of the displacement due to thermal expansion with a strain gauge or sensor. The m/z values in mass spectrum data are corrected based on the measured values of the displacement, whereby a highly accurate mass discrepancy correction is achieved.