The object of the invention is to provide a three-terminal switch (electrochemical transistor) capable of achieving sharp on-off operation.A source electrode and a drain electrode are juxtaposed with an insulator interposed between them, and on the assembly there is an ion diffusion member such as Ta2O5 located. On the opposite surface of the ion diffusion member, there is a gate electrode located that is capable of supplying metal ions such as copper ions. By application of voltage to the gate electrode, the metal ions going out of the gate electrode are reversibly precipitated as metal on both source and drain electrodes as well as on the insulator near them, thereby controlling conduction and non-conduction between the source electrode and the drain electrode.