Abstract: Composite resin containing cellulose is irradiated with infrared light, reflected light from the composite resin irradiated with the light is received, normalization is performed at a peak position maximized in a peak at 2800 cm?1 or more and 3000 cm?1 or less, which is a C—H stretching peak caused by the composite resin, in a reflection or absorption spectrum obtained by the reflected light, and a reflection or absorption spectrum for determination is obtained. The spectrum is used to acquire a ratio value of a spectral intensity (background intensity) at a position of 1000 cm?1 or less according to a determined resin type and different from a wave number at which a peak derived from resin of the determined resin type is expressed, and a ratio of the spectral intensity (background intensity) is used so that a composite of cellulose combined in composite resin can be determined with high accuracy.

Abstract: Composite resin containing cellulose is irradiated with infrared light, reflected light from the composite resin irradiated with the light is received, normalization is performed at a peak position maximized in a peak at 2800 cm?1 or more and 3000 cm?1 or less, which is a C—H stretching peak caused by the composite resin, in a reflection or absorption spectrum obtained by the reflected light, and a reflection or absorption spectrum for determination is obtained. The spectrum is used to acquire a ratio value of a spectral intensity (background intensity) at a position of 1000 cm?1 or less according to a determined resin type and different from a wave number at which a peak derived from resin of the determined resin type is expressed, and a ratio of the spectral intensity (background intensity) is used so that a composite of cellulose combined in composite resin can be determined with high accuracy.