PURPOSE: To relatively inexpensively produce a synthetic resin material having an excellent low-temp. radiation function by immersing a synthetic resin into an aq. tin chloride soln., then immersing this resin into an aq. platinum compd. soln., thereby precipitating platinum.

CONSTITUTION: The synthetic resin molded article or synthetic resin film is cleaned by washing away deposits, such as stains and fats and oils, at need, and is then immersed into the aq. tin chloride soln. The aq. tin chloride soln. is prepd. by dissolving about 5 to 40g/l tin chloride (II) in an aq. soln. of hydrochloric acid or sulfuric acid. The article to be treated is then washed with water and is immersed into the aq. platinum compd. soln. The aq. platinum compd. soln. preferably contains 0.008 to 0.08g/l platinum compd., such as platinum chloride. The adequate immersion conditions thereof are usually about 0.5 to 5 minutes at about 8 to 50°C. As a result, the platinum is uniformly precipitated on the article to be treated and the synthetic resin material which can maintain the uniform and excellent low-temp. radiation function over a long period of time is obtd.