The present invention provides an electrode active substance that is obtained by coating the surface of LiwAxDO4 particles (wherein A is at least one selected from the group consisting of Mn and Co, and D is one, two or more selected from the group consisting of P, Si and S, 0<w<=4, and 0<x<=1.5) with a coating layer comprising LiyEZPO4 (wherein E is at least one selected from the group consisting of Fe and Ni, 0<y<=2, and 0<z<=1.5) and a carbonaceous electron-conducting substance. This electrode active substance is obtained by suspending particles formed from LiwAXDO4 in an aqueous solution containing an Li source, E source, PO4 source and carbon source to obtain a slurry, then drying the slurry, and heat treating the product in a non-oxidizing atmosphere.