This invention provides a complex oxide comprising the features of: (i) being represented by the formula: (A0.4B0.1M0.1)x/0.6Co2Oy wherein A and B are elements differing from each other, each represents Ca, Sr or Ba, M represents Bi, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb or Lu, 1.7≦x≦2, and 3.8≦y≦5, (ii) having a Seebeck coefficient of 100 &mgr;V/K or more at a temperature of 100 K (absolute temperature) or higher and (iii) having an electrical resistivity of 10 m&OHgr;cm or less at a temperature of 100 K (absolute temperature) or higher. The complex oxide of the invention is a material composed of low-toxicity elements existing in large amounts, the material having superior heat resistance and chemical durability and a high thermoelectric conversion efficiency in a temperature range of 600 K or higher which falls in the temperature range of waste heat.