To simply manufacture a lithium-containing oxide at lower manufacturing cost. A method for manufacturing a lithium-containing composite oxide expressed by a general formula LiMPO4 (M is one or more of Fe (II), Mn (II), Co (II), and Ni (II)). A solution containing Li and P is formed and then is dripped in a solution containing M (M is one or more of Fe (II), Mn (II), Co (II), and Ni (II)) to form a mixed solution. By a hydrothermal method using the mixed solution, a single crystal particle of a lithium-containing composite oxide expressed by the general formula LiMPO4 (M is one or more of Fe (II), Mn (II), Co (II), and Ni (II)) is manufactured.