1. An apparatus for measuring temperature distribution in a target body, comprising: nuclear magnetic resonance imaging means for collecting chemical shift data from the target body at each voxel in an imaging target region in the target body, with and without a temperature change of the target body using a phase mapping imaging sequence; calculation means for calculating a difference between the chemical shift data collected with the temperature change and the chemical shift data collected without the temperature change at each voxel as a phase difference at each voxel between the chemical shift data collected with and without the temperature change; image construction means for constructing a temperature distribution image according to the difference calculated by the calculation means at each voxel by converting the phase difference at each voxel calculated by the calculation means into a corresponding temperature; and display means for displaying the temperature distribution image constructed by the image construction means.