To use aggregate of composite oxide comprising a metal such as Na, K, Rb, a metal such as Co, Fe, Ni, Cu and oxygen as a thermoelectric conversion material of high thermoelectric conversion rate safely at a low cost by orientating a c-axis of crystalline axis of oxide crystal in the same direction.

The composite oxide aggregate is one of composite oxide comprising at least one kind of metal selected from a group consisting of Na, K and Rb, at least one kind of metal selected from a group consisting of Co, Fe, Ni and Cu and oxygen. Performance of a material constituted of composite oxide with characteristic of anisotropy can be improved by making crystalline direction thereof uniform. Although composite oxide includes all what consist of above chemical species, NaCo2O4 is especially desirable from the view point of handling and cost.