It was demonstrated that the MgIn2O4 spinel is a very promising material as a transparent electronic conductor. By the measurements of diffuse reflectance spectra, the optical band gap of MgIn2O4 (approximately 3.4 eV) was found to be wider than that of ITO (indium tin oxide). Electrical conductivity of the sintered sample of MgIn2O4 at room temperature has reached almost 10(2) S cm-1 with no intentional doping. The conduction was found to be due to electrons introduced from oxygen vacancies.