The present invention provides a complex oxide having a composition represented by the formula La.sub.1-xM.sub.xNiO.sub.2.7-3.3 or (La.sub.1-xM.sub.x).sub.2NiO.sub.3.6-4.4 (wherein M is at least one element selected from the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi and Y, and 0.01.ltoreq..times..ltoreq.0.8), the complex oxide having a negative Seebeck coefficient at 100.degree. C. or higher. This complex oxide is a novel material which exhibits excellent properties as an n-type thermoelectric material.