1. A method comprising the steps of: when a local temperature change takes place in a certain position inside an object, acquiring a measured phase distribution image representing a temperature distribution inside said object using, as a temperature indicator, a phase of complex magnetic resonance signals from water protons inside said object observed by a magnetic resonance tomographic imaging technique; defining a certain position in said acquired measured phase distribution image as a region of interest; estimating a phase distribution of complex magnetic resonance signals before a temperature change in said region of interest based on a phase distribution of complex magnetic resonance signals in a portion surrounding said region of interest; acquiring an estimated phase distribution image based on said estimated phase distribution; calculating an amount of phase variation of complex magnetic resonance signals caused by a temperature change in said region of interest by conducting subtraction between said measured phase distribution image and said estimated phase distribution image on a pixel-by-pixel basis; and measuring an amount of a temperature change in said region of interest based on said amount of variation.