The far-infrared ray (FIR) has been applied to various fields such as medical therapy, kitchen utensils, bath supplies, and so on. The FIR-emitting agent was used to make functional paperboards to have freshness-maintaining ability. The FIR-emitting agent was diluted with different concentrations at 0.5% starch solution, and the FIR-emitting solutions were coated on paperboards, i.e., liner. The more the concentration of the FIR radiating agent increased at 0.5% cationic starch solution, the higher FIR emissivity and emission power of paperboards increased. The corrugated boxes made of paperboards coated by the FIR-radiating agents at over 5% dilution concentration endowed mandarin oranges in the boxes with greater antimicrobial activity than those in boxes made of paperboards coated by the agent at below 5% concentration. In addition, it was ascertained that treatment of the FIR agents rarely affected strength properties of paperboards.