We report on a metal-insulator transition in the LaAlO3-SrTiO3 interface electron system, of which the carrier density is tuned by an electric gate field. Below a critical carrier density nc ranging from 0.5 to 1.5×1013/cm2, LaAlO3-SrTiO3 interfaces, forming drain-source channels in field-effect devices, are nonohmic. The differential resistance at zero channel bias diverges within a 2% variation of the carrier density. Above nc, the conductivity of the ohmic channels has a metal-like temperature dependence, while below nc conductivity sets in only above a threshold electric field. For a given thickness of the LaAlO3 layer, the conductivity follows a σ0∝(n-nc)/nc characteristic. The metal-insulator transition is found to be distinct from that of the semiconductor 2D systems.