A fluid composition analysis mechanism includes a light source for applying excitation light to a sample fluid at a measurement position, a light receiving unit configured to receive and disperse Raman scattering light generated from the sample fluid irradiated with the excitation light, a Raman scattering light collection optical system configured to collect the Raman scattering light generated at the measurement position and to cause the Raman scattering light to be incident on the light receiving unit, a calculation unit for calculating a composition of the sample fluid based on an output of the light receiving unit, and a light shielding member arranged on the optical path of the excitation light or on the extended line of the excitation light.
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