NOVELTY - A metal salt is dissolved to obtain an aqueous solution of metal salt. The glycolic acid, citric acid, malic acid, maleic acid, malonic acid, fumaric acid, succinic acid, tartaric acid and/or their salt are dissolved to obtain an aqueous solution of carboxylic acid. The aqueous solution of carboxylic acid and aqueous solution of metal salt, or an aqueous solution of reducing agent are mixed to obtain a liquid mixture. The aqueous solution of metal salt or aqueous solution of reducing agent is added to the liquid mixture and mixed to produce metal nanoparticles.    USE - Synthesis of metal nanoparticles used for manufacturing metal film and metal nanoparticle dispersion liquid, and formation of electrode for solar cells (all claimed).    ADVANTAGE - The method enables manufacture of metal nanoparticles efficiently. The metal nanoparticle dispersion liquid formed using the metal nanoparticles has excellent dispersion stability.    DETAILED DESCRIPTION - A metal salt is dissolved to obtain an aqueous solution of metal salt. The glycolic acid, citric acid, malic acid, maleic acid, malonic acid, fumaric acid, succinic acid, tartaric acid and/or their salt are dissolved to obtain an aqueous solution of carboxylic acid. The aqueous solution of carboxylic acid and aqueous solution of metal salt, or an aqueous solution of reducing agent are mixed to obtain a liquid mixture. The aqueous solution of metal salt or aqueous solution of reducing agent is added to the liquid mixture and mixed to produce metal nanoparticles. The metal salt is mixed with the aqueous solution of reducing agent at 25-95 degrees C. The metal salt contains metallic element such as silver (75 mass% or more). The metal nanoparticles contain gold, platinum, palladium and/or ruthenium. The reducing agent is hydrazine, ascorbic acid, oxalic acid, formic acid and/or their salt, or sodium borohydride, potassium borohydride and/or glucose. An additive is an anion such as bromine ion, borate ion and/or phosphate ion or an organic compound such as carboxylic acid, polyvinyl pyrrolidone, polyvinyl pyrrolidone copolymer, cellulose ether, polyvinyl alcohol and/or polyacrylic acid is mixed with a dispersion medium to obtain metal nanoparticles dispersion liquid in an oxygen-free atmosphere. The dispersion medium contains water and alcohol. The alcohol is methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, diethylene glycol, isobornyl hexanol, glycerol and/or erythritol. A base material of silicon, glass, ceramic containing transparent electroconductive material, polymeric material or metal is baked to obtain electrode for solar cell. The metal salt is salt of barium, lead, bismuth, cerium, sodium, magnesium, potassium, calcium, aluminum, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, indium and/or tin.