Abstract: Provided is a gas detection device that is compact and can perform accurate measurement. The gas detection device includes a light-emitting section (10), a light-receiving section (20), and a light-guiding section (30) that guides light from the light-emitting section (10) to the light-receiving section (20). The light-guiding section (30) includes a mirror (50) and has a shape of part of at least one spheroid. The mirror (50) is provided at a position of or in proximity to a first focal point of the spheroid. The light-emitting section (10) and the light-receiving section (20) are each provided at a position of or in proximity to a focal point of the spheroid that is not the first focal point. The light-emitting section (10) and the light-receiving section (20) are arranged in parallel to a major axis direction of the spheroid.