In situ crystallized 20 nm thick SrTiO3 (STO) thin films were grown on a Ru substrate by atomic layer deposition. The leakage current-voltage characteristics of the film were examined from 313 to 403 K under electron injection conditions from the Ru electrode. The leakage current was dominated by Schottky emission in the low electric field (0.3 MV/cm) region. The estimated effective mass of the tunneling electron was similar to 0.1m(0).