A carbon-containing lithium-iron composite phosphorus oxide for a lithium secondary battery positive electrode active material, includes particles being composed of a lithium-iron composite phosphorus oxide having an olivine structure whose basic composition is LiFePO₄, and being composited with carbonaceous fine particles. A process for producing the same includes the steps of mixing a lithium compound making a lithium source, an iron compound making an iron source, a phosphorus-containing ammonium salt making a phosphorus source and carbonaceous fine particles, thereby preparing a mixture, and calcicing the mixture at a temperature of from 600 C. or more to 750 C. or less.