A resin is irradiated with an infrared ray, and a reflected ray from the resin irradiated with the infrared ray is received. In a reflection or absorption spectrum obtained by the reflected ray, a difference of a reflection intensity in a spectrum between a first wave number band of 1340 cm−1 to 1350 cm−1, inclusive, and a second wave number band of 1300 cm−1 to 1340 cm−1, inclusive, is calculated. It is determined whether or not a specific bromine-based flame retardant is contained in the resin, by using the calculated difference of reflection intensity in the spectrum.