A positive electrode active substance for a secondary cell, where the positive electrode active substance is capable of suppressing adsorption of water effectively in order to obtain a high-performance lithium ion secondary cell or sodium ion secondary cell. The positive electrode active substance contains 0.3 to 5 mass % of graphite, 0.1 to 4 mass % of carbon obtained by carbonizing a water-soluble carbon material, or 0.1 to 5 mass % of a metal fluoride is supported on a composite containing a compound which contains at least iron or manganese, where the compound is represented by formula (A) LiFeaMnbMcPO4, formula (B) Li2FedMneNfSiO4, or formula (C) NaFegMnhQiPO4, and carbon obtained by carbonizing a cellulose nanofiber.