The authors have fabricated a field-effect transistor (FET) based on a rutile Ti O2 active channel. Top-gate transistor structure with an amorphous LaAl O3 insulator was fabricated on the ultrasmoothed rutile single crystals. Reproducible n -type FET actions were observed only by the use of ultrasmoothed Ti O2 surfaces. Moreover, an anisotropy of the field-effect mobility depending on the channel direction, which reflects intrinsic nature of rutile, was definitely observed. Inserting MgO insulating buffer between Ti O2 and amorphous LaAl O3 suppressed the off-state current and realized on-to-off current ratio as high as 104.