Plastic bottles, containers, and packaging materials are commonplace items that can be found in various aspects of our everyday lives. In Japan, such items are discarded by consumers as waste plastic but are then collected and recycled into new raw materials and products according to the Container and Packaging Recycling Law in order to reduce and recycle waste plastic. The main four components of recycled plastics are polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), which are all commodity resins. Quality standards are defined for recycled plastics and to determine the composition of components, generally a sample is dissolved in a solvent and then analyzed using a nuclear magnetic resonance (NMR) spectrometer. In place of this general method, this article introduces a screening analysis method using a Fourier transform infrared spectrophotometer (FTIR). Quantitation is possible by either the individual calculation method, in which the concentration of each component is calculated individually, or the mixing ratio calculation method, in which the concentration of each component is calculated by taking the sample as a whole as 100 %. This article studies the mixing ratio calculation method. A major feature of this method is that unlike the precision analysis using an NMR spectrometer, sample pretreatment is unnecessary and therefore speedy calculations of the composition of components are possible.