A thermoelectric conversion material is a polycrystalline material composed of a plurality of crystal grains and has a composition represented by formula (I): Mg3+mSbaBi2−a−cAc. In the formula (I), A is at least one element selected from the group consisting of Se and Te, the value of m is greater than or equal to 0.01 and less than or equal to 0.5, the value of a is greater than or equal to 0 and less than or equal to 1.0, and the value of c is greater than or equal to 0.001 and less than or equal to 0.06. The thermoelectric conversion material has an Mg-rich region.