Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown(1-4). Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte(5). Sheet carrier density was enhanced from zero to 10(14) cm(-2) by applying a gate voltage of up to 3.5V to a pristine SrTiO3 single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals(6), indicating this method as promising for searching for unprecedented superconducting states.