A plastic identifying method of the present invention is provided with a step (i) of obtaining a first infrared absorption spectrum by irradiating infrared light of a predetermined wave number onto an item to be identified that contains plastic and measuring the intensity of the infrared light that is totally reflected by this item, and a step (ii) of identifying the plastic contained in the item to be identified by matching the first infrared absorption spectrum with a group of infrared absorption spectra that have been measured for predetermined materials, wherein the predetermined materials are a group of materials containing plastics, and each infrared absorption spectrum in this group of infrared absorption spectra is obtained by irradiating infrared light of a predetermined wave number onto a predetermined material, and measuring the intensity of the infrared light that is totally reflected by that material, and matching is carried out by comparing peaks in the first infrared absorption spectrum with peaks in each infrared absorption spectrum in the group of infrared absorption spectra. With this plastic identifying method, items that contain plastic can be identified accurately, even in such cases as when the items have been colored or contain additives or the like.