A liquid sample is irradiated with excitation light and measurement light, and a measurement position at which a traveling path of the measurement light passes through an excitation section of the excitation light in the sample is changed while the sample is being irradiated with the excitation light and the measurement light. Then, the phase change of the measurement light is measured for each measurement by optical interferometry on the basis of the measurement light after the measurement light passes through the sample. The measurement position is changed by, for example, scanning the excitation light, moving the sample, moving a lens that collects the excitation light in the sample so as to change the light-collecting position (focal position) in the sample, etc.
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