Li+ implantation at room temperature of insulating thin films of polycrystalline MgIn2O4 spinel (∼1.3 μm thick) was found to generate electron carriers efficiently. Li+ ions were implanted at 80 keV to a fluence of 1×1016 cm-2 and subsequently at 160 keV to the same fluence. Some implanted films were subjected to a post-annealing at 300 °C. Depth profiles of implanted Li+ ions measured with secondary-ion-mass spectroscopy agreed with that calculated with the trim code. Conductivity at room temperature increased from σ<10-7 to ∼101 S cm-1 upon the Li+ implantation. The generation yield of electron carriers in the as-implanted film was ∼20% and increased up to ∼40% upon post-annealing. Two optical-absorption bands were induced upon the implantation, one at about ∼500 nm and another above ∼1000 nm extending to the IR region, which was attributed to plasma oscillation of electron carriers. The former band faded and the latter absorption increased its intensity upon post-annealing. He + implantation, which was done for comparison, induced no change in electrical conductivity and no absorption band above ∼1000 nm.