A method for producing a thermoelectric conversion material composed of a metal A having an alkali metal or alkaline earth metal, a transition metal M, and oxygen O, and represented by AxMyOz, where x, y, and z are valences of the respective elements, includes the steps of: using a massive metal oxide as the thermoelectric conversion material and a salt in a solid, liquid or gaseous state; causing a diffusion reaction between the oxide and the salt; and forming the thermoelectric conversion material having aligned crystal orientation. A production apparatus includes a reactor into which the oxide and the salt are introduced, and a heating means for heating the oxide and the salt within the reactor to promote the diffusion reaction. Thereby, the thermoelectric conversion material having efficiency is produced more simply and at lower cost than a production of the single crystal.