Ammonium hexachlororuthenate is produced by adding ammonium chloride to a hydrochloric acid solution containing ruthenium. The ammonium hexachlororuthenate is baked to obtain the ruthenium powder. When the moisture content of the ammonium hexachlororuthenate is high, the baked product is so hard sintered product that its pulverization is not easy. In accordance with the present invention, the following steps are carried out. Hydrochloric acid solution containing ruthenium is held at a temperature of 80 to 95.degree. C. for three hours or longer. The ammonium chloride is then added to the hydrochloric acid solution which is stirred by a stirring mill at the rotation of 200 revolutions per minute or more. The hydrochloric acid solution is held at a temperature of from 85 to 95.degree. C. for 1 hour while being stirred at 200 rpm. The resultant precipitate of ammonium hexachlororuthenate is filtered. The inventive crystals of precipitated ammonium hexachlororuthenate has 10 mass % or less of moisture content.