TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The method for preparing the platinum nanoparticles involves mixing a chemical compound composed of at least one of elements of alkali metals and alkali earth metals with a solution of a platinum compound to form a mixture, and reducing the mixture with a reducing agent by blowing hydrogen gas in the mixture to bubble the same; or preparing a carbon support, a dispersion medium, a platinum compound having platinum, an adsorbent having at least one of elements of alkali metals and alkali earth metals, which is adsorbable on surfaces of the carbon support, and a capping reagent capable of capping specific areas of crystals of platinum; a first mixture liquid forming step of forming a mixture liquid in which the dispersion medium, the carbon support, the platinum compound and the adsorbent are mixed with each other; and a second mixture liquid forming step of forming a second mixture liquid in which the first mixture liquid and the capping reagent are mixed with each other; and reducing the second mixture liquid with the reducing agent to form platinum nanoparticles and support the platinum nanoparticles on the carbon support. In the method, the first mixture liquid is formed by mixing one part of the adsorbent in the dispersion medium in which the carbon support has been dispersed, by mixing the platinum compound in the dispersion liquid, and by mixing a remaining part of the adsorbent in the dispersion liquid; or by mixing the platinum compound in the dispersion liquid in which the carbon support has been dispersed, and thereafter by mixing the adsorbent in the dispersion medium. The chemical compound and mixture is not composed of sodium polyacrylate. Preferred Components: The chemical compound and adsorbent have at least one of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba). The chemical compound and adsorbent includes at least one of iodide, chloride and bromide. The solution of a platinum compound has potassium tetrachloroplatinate (K2PtCl4). The chemical compound has at least one of platinic acid chloride, potassium tetrachloroplatinate, diammine dinitro platinum, hexaammine platinum tetrachloride, tetraammine platinum dichloride, platinum chloride and dihydrogen chloroplatinate hydrate.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The capping reagent is composed of at least one of sodium polyacrylate, polyvinylpyrrolidone, polyvinyl alcohol, polyacrylamide, polyethyleneimine and polyethylene oxide. EXAMPLE - No suitable example is given.