We construct a device with transistor characteristics, taking advantage of the high ion conductivity of polymer solid electrolyte and electrochemical reversibility of conductive polymer. The device operates under a principle of operation whereby electrochemical oxidation and reduction of the conductive polymer are controlled by the gate voltage and current between the source and drain is modulated by changing the electronic state in the semiconductor layer. Poly(3-hexylthiophene) and cyanoethylpullulan are used as Polymer solid electrolyte and conductive polymer, respectively. Electric measurements show that the device has p -type transistor properties and a switching speed of 50 Hz.