A fluid composition analysis apparatus comprising: a light source configured to apply 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 arranged on an optical path of the excitation light or on an extended line of the excitation light and 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 configured to calculate a composition of the sample fluid based on an output of the light receiving unit; a light shielding member (18, 19) arranged on the optical path of the excitation light or on the extended line of the excitation light; a first measurement window arranged on the optical path of the excitation light and configured to guide the excitation light to an area in which the sample fluid flows; and a second measurement window arranged on the optical path of the excitation light or on the extended line of the excitation light and configured to guide the Raman scattering light generated at the measurement position to the Raman scattering light collection optical system arranged outside the area in which the sample fluid flows, characterized in that the light shielding member (18, 19) includes: a first light shielding member arranged on the side of the light receiving unit with respect to a surface of the second measurement window exposed to the sample fluid; and a second light shielding member arranged on the side of the light receiving unit with respect to the first light shielding member and having an outline defining the edge of the light blocking area coinciding with the outline of the first light shielding member, when viewed from the measurement point. a light source configured to apply 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 arranged on an optical path of the excitation light or on an extended line of the excitation light and 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 configured to calculate a composition of the sample fluid based on an output of the light receiving unit; a light shielding member (18, 19) arranged on the optical path of the excitation light or on the extended line of the excitation light; a first measurement window arranged on the optical path of the excitation light and configured to guide the excitation light to an area in which the sample fluid flows; and a second measurement window arranged on the optical path of the excitation light or on the extended line of the excitation light and configured to guide the Raman scattering light generated at the measurement position to the Raman scattering light collection optical system arranged outside the area in which the sample fluid flows, characterized in that the light shielding member (18, 19) includes: a first light shielding member arranged on the side of the light receiving unit with respect to a surface of the second measurement window exposed to the sample fluid; and a second light shielding member arranged on the side of the light receiving unit with respect to the first light shielding member and having an outline defining the edge of the light blocking area coinciding with the outline of the first light shielding member, when viewed from the measurement point. a first light shielding member arranged on the side of the light receiving unit with respect to a surface of the second measurement window exposed to the sample fluid; and a second light shielding member arranged on the side of the light receiving unit with respect to the first light shielding member and having an outline defining the edge of the light blocking area coinciding with the outline of the first light shielding member, when viewed from the measurement point.