A phosphorus-containing coated magnesium oxide powdered material comprising a coated magnesium oxide powdered material having a surface coating layer comprised of a double oxide, wherein the phosphorus-containing coated magnesium oxide powdered material has a coating layer comprised of a magnesium phosphate compound in at least part of a surface of the coated magnesium oxide powdered material, wherein the content of the magnesium phosphate compound in the coated magnesium oxide powdered material is 0.1 to 10% by mass, in terms of phosphorus; a method for producing a phosphorus-containing coated magnesium oxide powdered material, wherein the method comprises treating a coated magnesium oxide powdered material having a surface coating layer comprised of a double oxide with a phosphorus compound, and then calcining the resultant powdered material at 300.degree. C. or higher to form a magnesium phosphate compound in at least a part of a surface of the coated magnesium oxide powdered material; and a resin composition comprising the powder.