A gas analysis device (1, 1a), comprising: a base having a plate shape, and including a concave portion open at one surface of the base; a window located at a side of the base at which the concave portion is open, the window including a first film and a second film stacked with each other, the second film having a different refractive index than the first film; an optical part (20,120) located at a side of the window opposite to the base side, the optical part including a light projector and a light receiver, or the optical part including a light projector/receiver; and an optical path length controller located between the base and the window, a thickness of the optical path length controller being controllable, the concave portion including a first sidewall oblique to a surface of the base at which the concave portion is open, and a second sidewall facing the first sidewall, the second sidewall being oblique to the surface at which the concave portion is open, an oblique direction of the second sidewall being opposite to an oblique direction of the first sidewall, when the optical part includes the light projector and the light receiver, the light projector is configured to irradiate light toward the first sidewall, and the light receiver is configured to convert light reflected by the second sidewall into an electrical signal, when the optical part includes the light projector/receiver, the light projector/receiver is configured to irradiate light toward the first sidewall, and convert reflected light from the first sidewall into an electrical signal after the irradiation of the light is stopped. a base having a plate shape, and including a concave portion open at one surface of the base; a window located at a side of the base at which the concave portion is open, the window including a first film and a second film stacked with each other, the second film having a different refractive index than the first film; an optical part (20,120) located at a side of the window opposite to the base side, the optical part including a light projector and a light receiver, or the optical part including a light projector/receiver; and an optical path length controller located between the base and the window, a thickness of the optical path length controller being controllable, the concave portion including a first sidewall oblique to a surface of the base at which the concave portion is open, and a second sidewall facing the first sidewall, the second sidewall being oblique to the surface at which the concave portion is open, a first sidewall oblique to a surface of the base at which the concave portion is open, and a second sidewall facing the first sidewall, the second sidewall being oblique to the surface at which the concave portion is open, an oblique direction of the second sidewall being opposite to an oblique direction of the first sidewall, when the optical part includes the light projector and the light receiver, the light projector is configured to irradiate light toward the first sidewall, and the light receiver is configured to convert light reflected by the second sidewall into an electrical signal, the light projector is configured to irradiate light toward the first sidewall, and the light receiver is configured to convert light reflected by the second sidewall into an electrical signal, when the optical part includes the light projector/receiver, the light projector/receiver is configured to irradiate light toward the first sidewall, and convert reflected light from the first sidewall into an electrical signal after the irradiation of the light is stopped. the light projector/receiver is configured to irradiate light toward the first sidewall, and convert reflected light from the first sidewall into an electrical signal after the irradiation of the light is stopped.