The invention relates to a method for manufacturing a composite positive electrode active material being a composite of a positive electrode active material and carbon nanotubes. The manufacturing method includes preparing an aqueous solution of a starting material of a positive electrode active material containing a starting material of the positive electrode active material, and an aqueous solution of solubilized carbon nanotubes containing the carbon nanotubes and a solubilizing material that is composed of a water-soluble polymer whose solubilization retention rate of carbon nanotubes does not decrease with rising temperature; and synthesizing a positive electrode active material-carbon nanotube composite by mixing the aqueous solution of a starting material of a positive electrode active material and the aqueous solution of solubilized carbon nanotubes, and performing hydrothermal synthesis.