A gas detection apparatus (100) includes a first layer (1) and a second layer (2) disposed opposite the first layer (1) in a predetermined direction (z-axis direction). The first layer (1) includes a light emitter that emits light and a light receiver that receives the light after the light passes through a waveguide. The second layer (2) includes a light input unit of the waveguide opposite the light emitter in the predetermined direction (z-axis direction) and a light output unit of the waveguide opposite the light receiver in the predetermined direction (z-axis direction). The gas detection apparatus (100) can be miniaturized.