A dynamic change detecting apparatus allows canceling the influence of environmental change or an individual difference between a plurality of laser elements so as to perform a stable detection. This apparatus has a laser 7, including a laser resonator 21 to 25, for emitting a laser beam while causing frequency modulation in accordance with dimensional change of the laser resonator; a partial reflection mirror 33 for splitting the laser beam emitted by the laser 7 into a plurality of split-beams and guiding the plurality of split-beams to a plurality of optical paths having mutually different optical path lengths respectively; a frequency shifter 35 for causing frequency shift in at least one of the plurality of split-beams; a lens 39 for combining the plurality of split-beams with each other to obtain interference light; an photodetector 11 for detecting the interference light to obtain an intensity signal; a demodulation unit 43 for demodulating the intensity signal to generate a demodulated signal; and an integration processing unit 45 for obtaining a signal corresponding to the dynamic change on the basis of the demodulated signal. IMAGE
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