A method for manufacturing a gas concentration calculation device including a housing made of a synthetic resin and configured to include a cylindrical portion including first and second openings, respectively, at axial both ends thereof, and first and second mirrors, respectively, arranged facing each other at the first and second openings to form an optical path of infrared light inside the cylindrical portion. The method includes a step of preparing a precursor including respective sticking-out portions made of a synthetic resin sticking more outward than respective installation positions of first and second mirrors in a cylindrical portion and a step of bonding the first and second mirrors to the respective installation positions by arranging the first and second mirrors, respectively, at the respective installation positions of the precursor and then bending the sticking-out portions while melting with heat to cause them to adhere to the first and second mirrors.