A gas concentration measurement device comprising: a light source ; a detector ; wherein the measurement device is configured to measure a gas concentration based on an absorbance of sample gas in a region between the light source that emits infrared light and the detector including a light-receiving portion that receives the infrared light; a waveguide member including a wave-guiding portion having a tubular inner peripheral surface , an entrance portion that is formed at one side of the wave-guiding portion and through which the infrared light from the light source is introduced, and an exit portion that is formed at the other side of the wave-guiding portion and guides the infrared light that has passed through the wave-guiding portion toward the detector; a rotating member that is rotatable around a rotating shaft that intersects an axial direction of the waveguide member; a first band pass filter and a second band pass filter that are provided on the rotating member and located on a pair of planes that intersect each other; and a rotational driving unit that rotates the rotating member around the rotating shaft, wherein the rotating member is rotated by the rotational driving unit so that the first band pass filter and the second band pass filter are selectively located at a transmitting position at which the infrared light guided out of the exit portion is transmitted toward the detector, and wherein, when a portion of the rotating member, the first band pass filter, or the second band pass filter, the portion having a maximum radius of gyration around the rotating shaft, is defined as a maximum radius portion , and when a rotation locus obtained by imaginarily rotating the maximum radius portion around the rotating shaft in a view along the rotating shaft is defined as a reference circle, the exit portion is located in the reference circle. a light source ; a detector ; wherein the measurement device is configured to measure a gas concentration based on an absorbance of sample gas in a region between the light source that emits infrared light and the detector including a light-receiving portion that receives the infrared light; a waveguide member including a wave-guiding portion having a tubular inner peripheral surface , an entrance portion that is formed at one side of the wave-guiding portion and through which the infrared light from the light source is introduced, and an exit portion that is formed at the other side of the wave-guiding portion and guides the infrared light that has passed through the wave-guiding portion toward the detector; a rotating member that is rotatable around a rotating shaft that intersects an axial direction of the waveguide member; a first band pass filter and a second band pass filter that are provided on the rotating member and located on a pair of planes that intersect each other; and a rotational driving unit that rotates the rotating member around the rotating shaft, wherein the rotating member is rotated by the rotational driving unit so that the first band pass filter and the second band pass filter are selectively located at a transmitting position at which the infrared light guided out of the exit portion is transmitted toward the detector, and wherein, when a portion of the rotating member, the first band pass filter, or the second band pass filter, the portion having a maximum radius of gyration around the rotating shaft, is defined as a maximum radius portion , and when a rotation locus obtained by imaginarily rotating the maximum radius portion around the rotating shaft in a view along the rotating shaft is defined as a reference circle, the exit portion is located in the reference circle. <EOS>