Abstract: The present invention provides a complex oxide having a composition represented by the formula LavM1wNixM2yOz; wherein M1 is at least one element selected from the group consisting of Na, K, Sr, Ca, Bi and Nd; M2 is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co and Cu; and the subscripts are numbers which respectively satisfy 0.5?v?1.2; 0?w?0.5; 0.5?x?1.2; 0.01 ?y?0.5; and 2.8?z?3.2. The complex oxide of the invention has a negative Seebeck coefficient and a low electrical resistivity, i.e., an electrical resistivity of 10 m?cm or less, at 100° C. or higher and is a novel material with an excellent performance as an n-type thermoelectric material.